Previously, only sync branches were merged. This makes regular git push
into a repository watched by the assistant auto-merge.
While this does hardcode an assumption about what the remote tracking
branch is named, which some unusual git configurations won't match,
git-annex sync already made the same assumption.
Also, changed behavior when a tracking branch like
refs/remotes/synced/not/master is received. When on the master branch,
that used to get merged into it, but it's the tracking branch for
not/master, so should only be merged in when on the not/master branch.
This commit was sponsored by Ewen McNeill.
4a92eac23e