GitLab

On lab.nexedi.com `git-backup pull` became slow, and most of the slowness
was tracked down to the following:

`git fetch`, before fetching data from remote repository, first checks
whether it already locally has all the objects remote advertises. This
boils down to running

	echo $remote_tips | git rev-list --quiet --objects --stdin --not --all

and checking whether it succeeds or not:

	https://git.kernel.org/pub/scm/git/git.git/commit/?h=4191c35671
	https://git.kernel.org/pub/scm/git/git.git/tree/builtin/fetch.c?h=v2.18.0-rc1-1-g6f333ff2fb#n925
	https://git.kernel.org/pub/scm/git/git.git/tree/connected.c?h=v2.18.0-rc1-1-g6f333ff2fb#n8

The "--not --all" in the query means that objects should be not
reachable from all locally existing refs and is implemented by linearly
scanning from tip of those existing refs and marking objects reachable
from there as "do not print".

In case of git-backup, where we have mostly master which ...