RefService.ListRefs: emit the `ALL` pseudo-ref
We choose to have it point to the null commit id, in case other callers than `gitaly-backup` would make use of it: semantically, unless there is a special rule, it should mean a no-op, and we can hope for it. This would make compatible all future callers that don't need `ALL` to really represent all changesets. The other ones we'd have to patch if possible. Of course the Gitaly comparison test has to be adapted to discard `ALL`. Closes heptapod#679