Backups: ensure presence of the GitLab branches state file
Once Mercurial native projects don't use hg-git any more, we will stop backuping the auxiliary Git repository (except perhaps if there's push mirroring to an external Git service).
If the GitLab branches state file has not been initialized at this point, we'll need to force its creation, because it won't be possible after restoration.
See py-heptapod#8 (closed) for more details.