Bring back Rugged implementation of CommitIsAncestor
This brings back some of the changes in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20095.
For users using Gitaly on top of NFS, accessing the Git data directly
via Rugged is more performant than Gitaly. This merge request introduces
the feature flag rugged_commit_is_ancestor
to activate the Rugged path.
Part of four Rugged changes identified in https://gitlab.com/gitlab-org/gitlab-ce/issues/57317.