Deprecate the ability to use an arbitrary Git installation in favor of bundled Git

To fully reap the benefits of deploying from Git's master branch we will want to make use of new Git features immediately. As such, we will want to tightly couple the Git version and Gitaly with one another so that we always know that Gitaly runs with that exact version. Like this, we can immediately start using new features once the auto-update process has rolled out a new Git version.

This mandates that we deprecate the ability to use a non-bundled version of Git.