Allow internal fetches to see all hidden references

When re-homing a repository, it was noted at https://gitlab.com/gitlab-org/gitlab/issues/35864 that not all objects were copied across, most notably keep-around refs (but there could be others).

Stan suggested we:

Allow internal fetches to see all hidden references (e.g. refs/keep-around/X, refs/merge-requests/Y, etc.) if requested. This may involve changes in gitlab-shell (e.g. gitlab-shell!150 (diffs)), Gitaly, and the internal API.