The Gitaly Team and the Build Team should synchronize on Git releases

Spawned from https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1831

Over time, all git operations in GitLab will go through Gitaly. For this reason it's important that Gitaly is being tested with the same version of git as we're shipping in Omnibus.

(Currently Gitaly is tested against git 2.8 -- our 'minimum' version -- and git 2.13 -- our recommended version).

It's possible to envision a scenario where we upgrade to a version of Git that isn't being tested in Gitaly and something breaks in production. While it's not likely, it's worth discussing.

The Gitaly team and the Build team should coordinate on upgrade git dependencies.

The current Gitaly git test matrix is here: https://gitlab.com/gitlab-org/gitaly/blob/master/.gitlab-ci.yml

Cross-reference: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2668