revlog: fix index_fast_rank (wip) As far as I can tell, rank is stored as a 32-bit big endian value, I'm not sure how grabbing the first byte can possibly work. I assume there's no test coverage here? cc @pacien Fixes: https://www.mercurial-scm.org/repo/hg/rev/e633e660158f Differential Revision: https://phab.mercurial-scm.org/D12376