revlog: return 0 for the fast_rank of nullrev...
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
