After discussion with al & rco, conclusion was default PR merging method
should be rebase-and-merge for cleaner history.
Add test for that scenario (w/ test for final DAG) and implement this
change.
The old "sync pr" thing is turning out to be a bust, while it
originally worked fine these days it's a catastrophe as the v4 API
performances seem to have significantly degraded, to the point that
fetching all 15k PRs by pages of 100 simply blows up after a few
hundreds/thousands.
Instead, add a table of PRs to sync: if we get notified of a
"compatible" PR (enabled repo & target) which we don't know of, create
an entry in a "fetch jobs" table, then a cron will handle fetching the
PR then fetching/applying all relevant metadata (statuses,
review-comments and reviews).
Also change indexation of Commit(sha) and PR(head) to hash, as btree
indexes are not really sensible for such content (the ordering is
unhelpful and the index locality is awful by design/definition).
Turns out PATCH /git/refs/:ref returns a 422 when the ref does not
exist, rather than the 404 I'd expected.
Also improve the error message by including the JSON body which tends
to be more descriptive/helpful than the reason for Github's API.
Maybe I should replace all raise_for_status() by printing the JSON
body instead...
