Make Gitaly the definitive source of git-core

Right now, the Git version is defined in various different projects. Next to Gitaly itself, there's also CNG, GitLab, Omnibus, GitLab build images and probably more I didn't encounter yet. Taking #2959 (closed) as an example, there's been 12 merge requests working towards that goal. This shows that

• Changing Git versions is a major undertaking that's error prone in case one of those projects is forgotten, leading to inconsistencies.
• Iterating quickly is simply impossible with so many dependencies. It's hard to just out a bugfix release to Git or provide a hotfix in case some important feature is broken.
• It's also a hassle for the administrator if he supplies the Git version himself, as he'll be forced to upgrade Git in case we bump our requirements.

All in all, I think we should consolidate all those places to a single, definitive source. To me, Gitaly would be the natural place for this to live, also because we already have a build-git target which could be used to implement this.