revlog: return 0 for the fast_rank of nullrev

By convention, the rank of the null revision is 0. This particular revision is
never "physically" stored in the changelog, so it is a special case.

For consistency, the value `None` is still being returned for revlogs which do
not store the fast_rank property for any revision.

Differential Revision: https://phab.mercurial-scm.org/D12208

mercurial/revlog.py

@@ -869,8 +869,10 b' class revlog(object):'
         the revlog which do not persist the rank.
         """
         rank = self.index[rev][ENTRY_RANK]
-        if rank == RANK_UNKNOWN:
+        if self._format_version != CHANGELOGV2 or rank == RANK_UNKNOWN:
             return None
+        if rev == nullrev:
+            return 0  # convention
         return rank
 
     def chainbase(self, rev):