cext: shut-up sign compare warnings

Differential Revision: https://phab.mercurial-scm.org/D9627

mercurial/cext/revlog.c

@@ -2612,7 +2612,7 b' static int index_init(indexObject *self,'
 		                "data does not support buffer interface");
 		return -1;
 	}
-	if (self->nodelen < 20 || self->nodelen > sizeof(nullid)) {
+	if (self->nodelen < 20 || self->nodelen > (Py_ssize_t)sizeof(nullid)) {
 		PyErr_SetString(PyExc_RuntimeError, "unsupported node size");
 		return -1;
 	}