cext: make revlog.c PY_SSIZE_T_CLEAN

Without this, Python 3.8 emits a deprecation warning, as using
int for # values is deprecated. Many existing modules use
PY_SSIZE_T_CLEAN, so this shouldn't be contentious.

I audited the file for all # formatters and verified we are
using Py_ssize_t everywhere now.

Differential Revision: https://phab.mercurial-scm.org/D6196

mercurial/cext/revlog.c

@@ -7,6 +7,7 b''
  the GNU General Public License, incorporated herein by reference.
 */
 
+#define PY_SSIZE_T_CLEAN
 #include <Python.h>
 #include <assert.h>
 #include <ctype.h>
@@ -1947,7 +1948,7 b' static int index_populate_nt(indexObject'
 static PyObject *index_partialmatch(indexObject *self, PyObject *args)
 {
 	const char *fullnode;
-	int nodelen;
+	Py_ssize_t nodelen;
 	char *node;
 	int rev, i;