Best way to get a revno & depth from a revision-id?

[Ian Clatworthy]

If I have a branch & revision_id, is there an easy way to get the
revno & depth?

I'm asking because I want "log one rev" to be quick.
We convert the -rx.y.z option to a revision_id and
seem to throw away the mapping? To get it back, it looks
like we need to rebuild the whole rev_id_to_renvo map?

branch.revision_id_to_revno() doesn't look right either.
The revno I get back from that won't be x.y.z?

Ian C.