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