Code review comment for ~racb/git-ubuntu:staging-branches

Ok, got through the main update_staging_branches(). Overview:
- this function is called after devel (tip) was already updated via update_devel_branches(). If that fails, try/except around _main_with_repo() catches it, and just closes the repo and exits.
- if there is no tip, don't create the staging branch
- only stable series, as defined by LP ("supported")
- if there is no staging ref, create it pointing at tip
- if there is a staging ref, and it points to tip currently, skip (nothing to do)
- if there is a staging ref, and it can be fast-forwarded to tip, do it. If not, log the fact, and continue

Tomorrow morning I'll look at the tests