revlog: fix wrong type of rank_unknown variable

We treat "rank" as an int everywhere, but declare rank_unknown as a
char.  On architectures where char is signed, that works out ok, but
when char is unsigned, rank_unknown is 255 instead of -1.

Differential Revision: https://phab.mercurial-scm.org/D12374

mercurial/cext/revlog.c

@@ -179,7 +179,7 b' static const long entry_cl2_offset_rank '
 /* next free offset: 73 */
 
 static const char comp_mode_inline = 2;
-static const char rank_unknown = -1;
+static const int rank_unknown = -1;
 
 static void raise_revlog_error(void)
 {