Absorb gitlab_git
Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/24374
I'd like to get this merged sooner rather than later, so no extra work is done on https://gitlab.com/gitlab-org/gitlab_git that would need to be backported. Right now, no cleanup or deduplication of the code has been performed; everything is left pretty much as it was so as not to disturb the beast.