Run 'git gc' every 10 pushes

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/14250

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/12620

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/254

Associated gitlab-shell MR:
https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/44

See merge request !3206