GitLab

This brings the following changes of git-backup:

https://lab.nexedi.com/kirr/git-backup/compare/0b8d834b...cc6ac54f

the most important of which is dramatical speedup of pull and 30-50% speedup of restore if bitmap index of backup repo is built and git is recent enough.

Please see the following commits for details:

https://lab.nexedi.com/kirr/git-backup/commit/899103bf
https://lab.nexedi.com/kirr/git-backup/commit/3efed898
https://lab.nexedi.com/kirr/git-backup/commit/0ab7bbb6

/cc @alain.takoudjou, @rafael, @jerome