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
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[10]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate