gitlab: v↑ git-backup

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

software/gitlab/software.cfg

@@ -204,7 +204,7 @@ go.importpath = github.com/pkg/errors
 [git-backup-repository]
 <= go-git-repository
 go.importpath = lab.nexedi.com/kirr/git-backup
-revision = 0b8d834bd00b19169f65d407eab8980076779b9d
+revision = cc6ac54f451dfa6e343d6340dcfa25aa6eac9565
 
 
 [git-backup]