Return the revision_id for a dotted revno.

Args:
  revno: a tuple like (1,) or (1,1,2)
  _cache_reverse: a private parameter enabling storage
   of the reverse mapping in a top level cache. (This should
   only be done in selective circumstances as we want to
   avoid having the mapping cached multiple times.)
Returns: the revision_id
:raises errors.NoSuchRevision: if the revno doesn't exist
N)