This commit revisits the commands set in order to make it more
regular, and limit inconsistent command-sets, although it includes
pseudo-command aliases for common tasks now removed from the core set.
Hard Errors
===========
The previous iteration of the commands set would ignore any
non-command term in a command line. This has been changed to hard
error (and ignoring the entire thing) if any command is unknown or
invalid.
This fixes inconsistent / unexpected interpretations where a user
sends a command, then writes a novel on the same line some words of
which happen to *also* be commands, leading to merge states they did
not expect. They should now be told to fuck off.
Priority Restructuring
----------------------
The numerical priority system was pretty messy in that it confused
"staging priority" (in ways which were not entirely straightforward)
with overrides to other concerns.
This has now being split along all the axis, with separate command
subsets for:
- staging prioritisation, now separated between `default`, `priority`,
and `alone`,
- `default` means PRs are picked by an unspecified order when
creating a staging, if nothing better is available
- `priority` means PRs are picked first when staging, however if
`priority` PRs don't fill the staging the rest will be filled with
`default`, this mode did not previously exist
- `alone` means the PRs are picked first, before splits, and only
`alone` PRs can be part of the staging (which usually matches the
modename)
- `skipchecks` overrides both statuses and approval checks, for the
batch, something previously implied in `p=0`, but now
independent. Setting `skipchecks` basically makes the entire batch
`ready`.
For consistency this also sets the reviewer implicitly: since
skipchecks overrides both statuses *and approval*, whoever enables
this mode is essentially the reviewer.
- `cancel` cancels any ongoing staging when the marked PR becomes
ready again, previously this was also implied (in a more restricted
form) by setting `p=0`
FWBot removal
=============
While the "forwardport bot" still exists as an API level (to segregate
access rights between tokens) it has been removed as an interaction
point, as part of the modules merge plan. As a result,
fwbot stops responding
----------------------
Feedback messages are now always sent by the mergebot, the
forward-porting bot should not send any message or notification
anymore.
commands moved to the merge bot
-------------------------------
- `ignore`/`up to` simply changes bot
- `close` as well
- `skipci` is now a choice / flag of an `fw` command, which denotes
the forward-port policy,
- `fw=default` is the old `ci` and resets the policy to default,
that is wait for the PR to be merged to create forward ports, and
for the required statuses on each forward port to be received
before creating the next
- `fw=skipci` is the old `skipci`, it waits for the merge of the
base PR but then creates all the forward ports immediately (unless
it gets a conflict)
- `fw=skipmerge` immediately creates all the forward ports, without
even waiting for the PR to be merged
This is a completely new mode, and may be rather broken as until
now the 'bot has always assumed the source PR had been merged.
approval rework
---------------
Because of the previous section, there is no distinguishing feature
between `mergebot r+` = "merge this PR" and `forwardbot r+` = "merge
this PR and all its parent with different access rights".
As a result, the two have been merged under a single `mergebot r+`
with heuristics attempting to provide the best experience:
- if approving a non-forward port, the behavior does not change
- else, with review rights on the source, all ancestors are approved
- else, as author of the original, approves all ancestors which descend
from a merged PR
- else, approves all ancestors up to and including the oldest ancestor
to which we have review rights
Most notably, the source's author is not delegated on the source or
any of its descendants anymore. This might need to be revisited if it
provides too restrictive.
For the very specialized need of approving a forward-port *and none of
its ancestors*, `review=` can now take a comma (`,`) separated list of
pull request numbers (github numbers, not mergebot ids).
Computed State
==============
The `state` field of pull requests is now computed. Hopefully this
makes the status more consistent and predictable in the long run, and
importantly makes status management more reliable (because reference
datum get updated naturally flowing to the state).
For now however it makes things more complicated as some of the states
have to be separately signaled or updated:
- `closed` and `error` are now separate flags
- `merge_date` is pulled down from forwardport and becomes the
transition signal for ready -> merged
- `reviewed_by` becomes the transition signal for approval (might be a
good idea to rename it...)
- `status` is computed from the head's statuses and overrides, and
*that* becomes the validation state
Ideally, batch-level flags like `skipchecks` should be on, well, the
batch, and `state` should have a dependency on the batch. However
currently the batch is not a durable / permanent member of the system,
so it's a PR-level flag and a messy pile.
On notable change is that *forcing* the state to `ready` now does that
but also sets the reviewer, `skipchecks`, and overrides to ensure the
API-mediated readying does not get rolled back by e.g. the runbot
sending a status.
This is useful for a few types of automated / programmatic PRs
e.g. translation exports, where we set the state programmatically to
limit noise.
recursive dependency hack
-------------------------
Given a sequence of PRs with an override of the source, if one of the
PRs is updated its descendants should not have the override
anymore. However if the updated PR gets overridden, its descendants
should have *that* override.
This requires some unholy manipulations via an override of `modified`,
as the ORM supports recursive fields but not recursive
dependencies (on a different field).
unconditional followup scheduling
---------------------------------
Previously scheduling forward-port followup was contigent on the FW
policy, but it's not actually correct if the new PR is *immediately*
validated (which can happen now that the field is computed, if there
are no required statuses *or* all of the required statuses are
overridden by an ancestor) as nothing will trigger the state change
and thus scheduling of the fp followup.
The followup function checks all the properties of the batch to port,
so this should not result on incorrect ports. Although it's a bit more
expensive, and will lead to more spam.
Previously this would not happen because on creation of a PR the
validation task (commit -> PR) would still have to execute.
Misc Changes
============
- If a PR is marked as overriding / canceling stagings, it now does
so on retry not just when setting initially.
This was not handled at all previously, so a PR in P0 going into
error due to e.g. a non-deterministic bug would be retried and still
p=0, but a current staging would not get cancelled. Same when a PR
in p=0 goes into error because something was failed, then is updated
with a fix.
- Add tracking to a bunch of relevant PR fields.
Post-mortem analysis currently generally requires going through the
text logs to see what happened, which is annoying.
There is a nondeterminism / inconsistency in the tracking which
sometimes leads the admin user to trigger tracking before the bot
does, leading to the staging tracking being attributed to them
during tests, shove under the carpet by ignoring the user to whom
that tracking is attributed.
When multiple users update tracked fields in the same transaction
all the changes are attributed to the first one having triggered
tracking (?), I couldn't find why the admin sometimes takes over.
- added and leveraged support for enum-backed selection fields
- moved variuous fields from forwardport to runbot_merge
- fix a migration which had never worked and which never run (because
I forgot to bump the version on the module)
- remove some unnecessary intermediate de/serialisation
fixes#673, fixes#309, fixes#792, fixes#846 (probably)
- move all commands parsing to runbot_merge as part of the long-term
unification effort (#789)
- set up an actual parser-ish structure to parse the commands to
something approaching a sum type (fixes#507)
- this is mostly prep for reworking the commands set (#673), although
*strict command parsing* has been implemented (cf update to
`test_unknown_commands`)
Not sure why I didn't think about it previously (in #357), but it
would make sense for the entire batch creation to be atomic and thus
under the same lock.
The commit during forward porting also makes a lot less sense: it was
a failed early attempt at resolving the problem by hoping we'd win the
race with the webhook (commit before the webhook hit). By locking the
PRs table to update, we actually resolved it.
But since all that happens then is a few updates and then a commit by
the cron itself (it commits per batch), it's probably good enough to
leave the entire thing under the same lock. This means we lock out
other interactions a bit longer, but since the span is still just the
forward port of a single batch it should not be too much of an issue
outside of post-freeze recovery thingie.
Continuation of 327500bc83 for an other
edge case of closing a PR to a detached branch with a merged
descendant. The mergebot would:
- warn on the parent about it being detached due to being closed
- then warn on the child about it being detached due to the parent
being closed (despite it being merged already)
- then warn the parent *again* due to the child being detached
At least some of those messages were still produced by the test case,
stop them.
Issue was noticed on odoo/odoo#145969 and odoo/odoo#145984 due to 16.2
being deactivated.
The notification is both noise and confusing: we're telling the
author (and reviewer, and anyone else subscribed) that they need to
merge a merged PR.
Fixes#855
When reparenting a commit (mostly when inserting a new forwardport in
an existing chain after a freeze / branch insertion), the new source
should be the source of the new parent (which is likely a not-change
of the source).
This was miscomputed to the root of the new parent, which often
matches but breaks if there was a conflict or a mid-port update,
leading to inconsistent presentation. Nothing critical, just somewhat
annoying.
Currently, once a source PR has been merged it's not possible to set
or update a limit, which can be inconvenient (e.g. people might have
forgotten to set it, or realise after the fact that setting one is not
useful, or might realise after the fact that they should *unset* it).
This PR relaxes that constraint (which is not just a relaxation as it
requires a bunch of additional work and validation), it should now be
possible to:
- update the limit to any target, as long as that target is at or
above the current forwardport tip's
- with the exception of a tip being forward ported, as that can't be
cancelled
- resume a forward port stopped by a previously set limit (just
increase it to whatever)
- set a limit from any forward-port PR
- set a limit from a source, post-merge
The process should also provide pretty chatty feedback:
- feedback on the source and closest root
- feedback about adjustments if any (e.g. setting the limit to B but
there's already a forward port to C, the 'bot should set the limit
to C and tell you that it happened and why)
Fixes#506
Currently a user is not notified that the parent of a detached PR
needs to be independently approved and may miss that information. Add
a notification to *that* PR as well.
Fixes#788
The github API has gotten a lot more constraining (with rate
restrictions being newly enforced or added somewhat out of nowhere),
and as importantly a lot less reliable. So move the staging process
off of github and locally, similar to the forward porting
process (whose repo cache is being reused for this).
Fixes#247
If the primary email is made public, it is returned directly as part
of the /users endpoint, in which case we don't need to fetch it via
/user/emails.
Also improve error messages, and fix the incorrect checks on the
existence of the github name and email. And allow manually updating
both via the project form.
Move *almost* all the staging code to free functions, in a separate
module, and extensively typed.
The only bits which didn't move are:
- the entry point (the cron hook), because it has to be a model method
in order to be called
- the `_build_merge_message` method, because it needs to be
overridable
There's also a bit of an import mess, because the cron &
`_build_merge_message` need to call into the new module, but the new
module wants the types they belong to, so it's a bit circular.
If the stagings are going to be created locally (via a git working
copy rather than the github API), the mergebot part needs to have
access to the cache, so move the cache over. Also move the maintenance
cron.
In an extermely minor way, this prefigures the (hopeful) eventual
merging of the ~~planes~~ modules.
In 81ce4ea02b the delta for PRs being
listed in the `/forwardport/outstanding` page was increased from 3
days to 7 (1 week), however the warning box in the home page still
used the old cutoffs leading to
An inconsistency between the two and an effective severe overcounting,
as the reason why the cutoff was increased to a week is forward ports
can take a while or the author / reviewer can be a touch busy at end
of week, so 3~4 days are routine when a PR is merged on thursday or
friday (and even worse if there's bank holidays in the mix).
Fix outstanding query to make a positive `state` filtering, instead of
negative, matching 3b52b1aace8674259812a76b1566260937dbcacb.
Also manually create a map of stagings (grouped by branch) sharing a
single prefetch set.
For odoo the mergebot home page has 12 branches in the odoo project
and 8 in spreadsheet, 6 stagings each. This means 120 queries to
retrieve all the heads (Odoo stagings have 5 heads and spreadsheet
have 1, but that seems immaterial).
By fixing `_compute_statuses` and creating a single prefetch set for
all stagings of all branches we can fetch all the commits in a single
query instead of 120.
- add support for authorship (not just approval)
- make display counts directly
- fix `state` filter: postgres can't do negative index lookups
- add indexes for author and reviewed_by as we look them up
- ensure we handle the entire source filtering via a single subquery
Closes#778
Currently sentry is only hooked from the outside, which doesn't
necessarily provide sufficiently actionable information.
Add some a few hooks to (try and) report odoo / mergebot metadata:
- add the user to WSGI transactions
- add a transaction (with users) around crons
- add the webhook event info to webhook requests
- add a few spans to the long-running crons, when they cover multiple
units per iteration (e.g. a span per branch being staged)
Closes#544
Current system makes it hard to iterate feedback messages and make
them clearer, this should improve things a touch.
Use a bespoke model to avoid concerns with qweb rendering
complexity (we just want GFM output and should not need logic).
Also update fwbot test setup to always configure an fwbot name, in
order to avoid ping messages closing the PRs they're talking
about, that took a while to debug, and given the old message I assume
I'd already hit it and just been too lazy to fix. This requires
updating a bunch of tests as fwbot ping are sent *to*
`fp_github_name`, but sent *from* the reference user (because that's
the key we set).
Note: noupdate on CSV files doesn't seem to work anymore, which isn't
great. But instead set tracking on the template's templates, it's not
quite as good but should be sufficient.
Fixes#769
I'd been convinced this was an ORM error because the field is not
recursive... in runbot_merge, in forwardbot it is and thus does indeed
need to be flagged to avoid the warning.
- currently disabling staging only works globally, allow disabling on
a single branch
- use a toggle
- remove a pair of tests which work specifically with `fp_target`,
can't work with `active` (probably)
- cleanup search of possible and active stagings, add relevant
indexes and use direct search of relevant branches instead of
looking up from the project
- also use toggle button for `active` on branches
- shitty workaround for upgrading DB: apparently mail really wants to
have a `user_id` to do some weird thing, so need to re-add it after
resetting everything
Fixes#727
I DECLARE BANKRUPTCY!!!
The previous implementation of labels lookup was really not
intuitive (it was just a char field, and matched labels by equality
including the owner tag), and was also full of broken edge
cases (e.g. traceback if a label matched multiple PRs in the same repo
because people reuse branch names).
Tried messing about with contextual `display_name` and `name_search`
on PRs but the client goes wonky in that case, and there is no clean
autocomplete for non-relational fields.
So created a view which reifies labels, and that can be used as the
basis for our search. It doesn't have to be maintained by hand, can be
searched somewhat flexibly, we can add new view fields in the future
if desirable, and it seems to work fine providing a nice
understandable UX, with the reliability of using a normal Odoo model
the normal way.
Also fixed the handling of bump PRs, clearly clearing the entire field
before trying to update existing records (even with a link_to
inbetween) is not the web client's fancy, re-selecting the current
label would just empty the thing entirely.
So use a two-step process slightly closer to the release PRs instead:
- first update or delete the existing bump PRs
- then add the new ones
The second part is because bump PRs are somewhat less critical than
release, so it can be a bit more DWIM compared to the more deliberate
process of release PRs where first the list of repositories involved
has to be set up just so, then the PRs can be filled in each of them.
Fixes#697
Previously the mergebot would only sync the head commit, but synching
more is useful.
Also update the final sanity check on staging:
- as with check, update the message & target branch
- reset PR state and post a message when updating message instead of
doing so silently
Note: maybe only fail the staging if the message is updated *and*
relevant to staging (aka there's a merge method and it's not
`rebase`)?
Fixes#680
Currently, if a PR forward-port PR gets detached the reason for it is
not always obvious, and may have to be hunted in the logs or in
"sibling" PRs.
By writing a forward port reason (hopefully) ever time we detach a PR,
and displaying that reason in the form and dashboard, the
justification should be a lot more obvious.
Fixes#679
When inserting a new branch, if there are extant forward ports
sequences spanning over the insertion, new forward ports must be
created for the new branch.
This was the case, *however* one of the cases was handled incorrectly:
PR batches (PRs to different repositories staged together) must be
forward-ported as batches. However when creating the "insert" batches,
these PRs would be split apart, leading to either inconsistent states
or an inability to merge the new forward ports (if they are
co-dependent which is a common case). This is because the handler
would look for all the relevant PRs to forward ports, but it would
fail to group them by label thereafter.
Fix that, create the `insert` batches correctly, and augment the
relevant test with an additional repository to test this case.
No gh issue was created, but this problem was reported on
odoo/odoo#110029 and odoo/enterprise#35838 (they were re-linked by
hand in the runbot and mergebot, but on github the labels remain
visibly different, one having the slug ZCwE while the other
hasO7Pr).
It's possible that the problem had occurred previously and not been
noticed (or reported), though it's also possible this issue had never
occurred before: for the latest freeze (16.1) there were 18 forward
ports spanning the insertion, but of them only 2 were the same batch.
It's almost certainly not useful, save as a minor convenience for
tests: decorrelating the branch sequence and the fp sequence seems
like it would only be extremely confusing, and on the mergebot all the
fp_sequence values are set to the default while the sequence values
are set to something useful and sensible (kinda).
Fixes#584
After review, there doesn't seem to be a single integer field created
by the mergebot or fortwardbot modules for which a `group_operator`
makes sense, let alone the default of `sum`.
So just disable them all.
Fixes#674
In case where the last branch (before the branch being frozen) is
disabled, the forwardport inserter screws up, and fails to correctly
create the intermediate forwardports from the new branch.
Also when disabling a branch, if there are FW PRs which target that
branch and have not been forward-ported further, automatically
forward-port them as if the branch had been disabled when they were
created, this should limit data loss and confusion.
Also change the message set on PRs when disabling a branch: because of
user conflicts in test setup, the message about a branch being
disabled would close the PRs, which would then orphan the followup,
leading to unexpected / inconsistent behaviour.
Fixes#665
Since the forwardport bot works off of PRs, when it was created
leveraging the magic refs of github (refs/pull/*/head) seemed
sensible: when updating the cache repo, the magic refs would be
updated alongside and the forward-porting would have all the commits
available.
Turns out there are a few issues with this:
- the magic refs have become a bit unreliable, so we need a fallback
(b1c16fce8768080d30430f4e6d3788b60ce13de7)
- the forwardport bot only needs the commits on a transient basis, but
the magic refs live forever and diverge from all other content
(since we rarely `merge` PRs), for a large repo with lots of PRs
this leads to a significant inflation in size of repo (both on-disk
and objects count) e.g. odoo/odoo has about 25% more objects
with the pull refs included (3486550 to 4395319) and takes nearly
50% more space on disk (1.21G to 1.77)
As a result, we can just stop configuring the pull refs entirely, and
instead fetch the heads (~refs) we need as we need them. This can be a
touch more expensive at times as it requires two `fetch` calls, and
we'll have a few redundant calls as every forward port of a chain will
require a fetch of the root's head, *however* it will avoid retrieving
PR data for e.g. prs to master, as they don't get forward-ported, they
also won't get parasite updates from PRs being ignored or eventually
closed.
Eventually this could be optimised further by
- performing an update of the cache repo at the start of the cron iff
there are batches to port
- creating a temp clone for the batch
- fetching the heads of all PRs to forward port in the temp clone in a
single `fetch`
- performing all the ports by cloning the temp clone (and not
`fetch`-ing into those)
- then cleaning up the temp clone after having cleaned up individual
forward port clones
Small followup for #489
The current system makes / lets GC run during fetching. This has a few
issues:
- the autogc consumes resources during the forward-porting
process (not that it's hugely urgent but it seems unnecessary)
- the autogc commonly fails due to the combination of large repository
(odoo/odoo) and low memory limits (hardmem for odoo, which get
translated into soft ulimits)
As a result, the garbage collection of the repository sometimes stops
entirely, leading to an increase in repository size and a decrease in
performances.
To mitigate this issue, disable the automagic gc and maintenance
during normal operation, and instead add a weekly cron which runs an
aggressive GC with memory limits disabled (as far as they can get, if
the limits are imposed externally there's nothing to be done).
The maintenance is implemented using a full lockout of the
forward-port cron and an in-place GC rather than a copy/gc/swap, as
doing this maintenance at the small hours of the week-end (sat-sun
night) seems like a non-issue: currently an aggressive GC of odoo/odoo
(using the default aggressive options) takes a total of 2:30 wallclock
(5h user) on a fairly elderly machine (it's closer to 20mn wallclock
and 2h user on my local machine, also turns out the cache repos are
kinda badly configured leading to ~30% more objects than necessary
which doesn't help).
For the record, a fresh checkout of odoo/odoo right now yields:
| Overall repository size | |
| * Commits | |
| * Count | 199 k |
| * Total size | 102 MiB |
| * Trees | |
| * Count | 1.60 M |
| * Total size | 2.67 GiB |
| * Total tree entries | 74.1 M |
| * Blobs | |
| * Count | 1.69 M |
| * Total size | 72.4 GiB |
If this still proves insufficient, a further option would be to deploy
a "generational repacking" strategy:
https://gitlab.com/gitlab-org/gitaly/-/issues/2861 (though apparently
it's not yet been implemented / deployed on gitlab so...).
But for now we'll see how it shakes out.
Close#489
- avoid pinging the author of the fw PR (which is the forward-bot
itself)
- instead ping the author and reviewer of the source, and possibly the
reviewer of the PR if any
- might also be a good idea to ping reviewers of intermediate PRs?
Github can fail to create the magic refs on PRs
(`pull/refs/?/head`). Since forwardport relies on these refs to fetch
PR content this is an issue when it occurs, as the forward ports fail
in a loop.
After discussion with Github support, it turns out Github enabled
`allowReachableSHA1InWant` a while back, meaning it's possible to
fetch content by commit (rather than ref) as long as the content is
"in network".
Use this property as fallback when checking if we can see the PR head
before forward porting.
Also:
- remove explicit configuration of GC during fetch, it doesn't disable
the autogc (yet?) but that's likely going to happen anyway
- update logging and logger hierarchy during forward port to make
things clearer and easier to extract, although based on PR id rather
than number
- rate limit failing forward ports to avoid running them on every cron
(~ every minute), run them every ~30mn instead, this provides higher
odds of recovery with less log garbage in case of transient github
failure, and if the PR is stuck it limits the log pollution
Fixes#658
If no stream data was captured (no stderr and no stdout), would just
log
git call error:
as error, with no further information.
Don't do that if we have neither stderr nor stdout data, since we're
re-raising the exception anyway, it's just confusing.
- if stderr was empty or had been redirected to stdout, no useful
information would be show, making debugging more complicated
- the fallback is the error itself, but since it's reraised odds are
pretty high the caller will eventually log the error itself, so
it's redundant
=> fallback to stdout if stderr is empty, and only log if either is
non-empty
Get mergebot updates from since the runbot was upgraded.
NOTE: updates forwardport.models.forwardport.Queue with slots for
compatibility with commit
odoo/odoo@ea3e39506a "use slots for
BaseModel", otherwise we get
TypeError: __bases__ assignment: 'ForwardPortTasks' object layout differs from 'BaseModel'
- if stderr has been rerouted or explicitely rerouted to STDOUT,
`e.stderr` is `None` and the error reporting blows up (which is
inconvenient). Handle this case.
- handle the case where `fp_github_name` has not been configured (it's
not a super useful handling but meh, apparently git/hub doesn't
really care if there's a username when using API tokens)
- minor improvement to the refline parser: per-spec, the trailing
newline is optional, so don't fail if it's missing
I'm surprised this ever worked, I guess concurrent tests stopped
working long before that? Or I misunderstood some of the historical
failures as transient?
During the cleanup of the forwardport test, I'd empty out the
`user_cache_dir('forwardport') / owner`, except the owner is always
the same (more or less) so all the tests check out their repos (and
working copies) in the same directory. If one test is cleaning up
while an other is performing a forward port, the second will blow up.
Also move the filestore to a tempdir, especially during creation of
the template db: it gets leaked so over time that generates gigabytes
of data which doesn't get cleaned up. But the template db filestore is
only "necessary" during the creation of the template, once the
template's been created it's of no use and won't be copied to create
the test dbs (though it could be, I guess).
Stop *staging* release PRs: they are normally fairly simple and should
not fail their staging outside of unreliable tests (or possibly a few
edge cases e.g. forgot one version change thing), however staging them
creates the possibility of a "version hole" on the release branch
which is undesirable.
Instead, immediately and unconditionally push the release commits onto
the newly created branches, if there are things which don't work they
can be fixed afterwards (and the process refined, maybe).
Also add the same feature for *bump* PRs, with the difference that the
bump PRs are not created / requested by default (they have to be opted
in individually).
For convenience, add a feature which automatically finds the PRs via
inputting the label (not really tested yet).
Closes#603
It's likely that the PR pages are seen more commonly than the
dashboard by most users, so add alerts there in case users wonder
what's happening.
Fixes#580
Check remains a tad dodgy, but since we're actually porting from
`root` (the earliest ancestor of `self`) it makes more sense to
sanity-check that *its* commits remain visible.
Also log that as it makes a tad more sense, hopefully.
Closes#600
This is an important bit of information but it was not visible without
going into the backend.
`user-select-none` doesn't work in BS3 but that way it'll be ready for
an eventual update. Currently when hovering the badge the cursor
switches to text selection, and the text is selectable, which is
useless.
Complements 4e235a2 and finishes the fixes for #617
- trying to r+ a detached PR *via the forwardbot* should warn, same as
a non-forwardport PR
- the following sibling of a closed PR should be detached from
it (probably)
- when a closed forward-port PR is reopened, there should be a
notification that it is detached and merged via mergebot
Fixes#617
Existing conflict style is the local default ("merge", most
likely). `diff3` is a lot more informative as it provides the common
ancestor's code for the hunk, which helps see how the two branches
diverged and thus resolve the conflict.
Even better would be zdiff3 but that's a bit too recent...
Fixes#619
