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

mercurial/cext/revlog.c

@@ -588,7 +588,7 b' static int index_fast_rank(indexObject *'
 		return 0; /* convention */
 	}
 
-	return *(index_deref(self, pos) + entry_cl2_offset_rank);
+	return getbe32(index_deref(self, pos) + entry_cl2_offset_rank);
 }
 
 /*