Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/33877

Regressed in
https://gitlab.com/gitlab-org/gitlab-ce/commit/5a95d6f8dae00b31b694759c6ddbf6d83b1a7890#3a15290e7696397415523e0b664aceb3dd4010ae_0_46
where the ref no longer matches what we call `renderGFM` on.