This brings the following changes of git-backup:

	kirr/git-backup@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:

	kirr/git-backup@899103bf
	kirr/git-backup@3efed898
	kirr/git-backup@0ab7bbb6

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

/reviewed-on nexedi/slapos!342