Switch to bundled Git

Gitaly has introduced a new mode which allows usage of a bundled Git version, which is installed in parallel to the usual Git binaries as "gitaly-git", "gitaly-git-http-backend" and "gitaly-git-remote-http". This is done to allow tighter control over the Git version used by Gitaly, allow feature flags and fix issues with overwriting Git distributions while Gitaly is running in zero-downtime upgrades. Distributions need to adapt to this new distribution mode.