Requires parsing the commit messages as github plain doesn't collate
the info from there, but also the descriptions: apparently github only
adds those to the references if the PR targets the default
branch. That's not a limitation we want.
Meaning at the end of the day, the only thing we're calling graphql
for is explicit manual linking, which can't be tested without
extending the github API (and then would only be testable on DC), and
which I'm not sure anyone bothers with...
Limitations
-----------
- Links are retrieved on staging (so if one is added later it won't be
taken in account).
- Only repository-local links are handled, not cross-repository.
Fixes#777
If a branch `foo` is disabled, then `tmp.foo` and `staging.foo` become
unnecessary (with #247 fixed the tmp refs are not used for creating
stagings anymore, but for now they're still used for the "safety
dance" of merging a successful staging into the corresponding
mainline).
Fixes#605
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.
