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.