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
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