GitlabReferenceCounter misuse

In !164 (merged) , GitlabReferenceCounter was altered to use gl_repository as the key instead of repo_path in the pre-receive and post-receive hooks - along with a fallback to the repo_path if the internal API returns a 404 (older version of GitLab).

However, users in gitlab-shell such as mv_storage still initialize the reference counter with repo_path. Having a reference counter that can increment / decrement two different references depending on external factors is not useful at all.

I'm not sure why it was changed to gl_repository but I think we should probably change it back to repo_path throughout. I probably haven't seen all the missed sites.

Will changes be needed in Gitaly as well?

/cc @eReGeBe @DouweM