`gc --prune` can not take a *separate* parameter, it has to be part of
the same arg (the `=` is not optional), otherwise the `gc` call blows
up.
So use the positional form of the git command to generate the correct
invocation, Python-level `foo=bar` generates a split-style option in
two args which does not please git.
Before this, we would check if a repository had a name and run
maintenance on it, leading to repeated (but unnoticed until now
because I didn't monitor it) tracebacks as the maintenance cron would
fail to find the local repo then run maintenance on nowhere anyway.
Also augment the repo-finding process to try and get better
information about what it's doing when it fails, rather than failing
completely silently.
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.
