The original method was to `git diff | git apply` in order to get a
complete overview of conflicts generated by the forward port (if
any).
However this turns out to have a huge issue in the presence of renamed
or removed files: in that case `git apply` will simply not do
anything, and fail with a completely clean working copy. Which is very
much undesirable.
-> alternative method, squash the PR to a single commit then
cherry-pick that single commit, this should provide us with proper
conflicts & their markers.
Also add tests for conflicts due to deleted files...
* Cherrypicking is handrolled because there seems to be no easy way to
programmatically edit commit messages during the cherrypicking
sequence: `-n` basically squashes all commits and `-e` invokes a
subprocess. `-e` with `VISUAL=false` kinda sorta works (in that it
interrupts the process before each commit), however there doesn't
seem to be clean status codes so it's difficult to know if the
cherrypick failed or if it's just waiting for a commit of this step.
Instead, cherrypick commits individually then edit / rewrite their
commit messages:
* add a reference to the original commit
* convert signed-off-by to something else as the original commit was
signed off but not necessarily this one
* Can't assign users when creating PRs: only repository collaborators
or people who commented on the issue / PR (which we're in the
process of creating) can be assigned.
PR authors are as likely to be collaborators as not, and we can have
non-collaborator reviewers. So pinging via a regular comment seems
less fraught as a way to notify users.
