reachableroots: pass NULL to PySet_New() as it expects a pointer, not an int

mercurial/parsers.c

@@ -1144,7 +1144,7 b' static PyObject *reachableroots(indexObj'
 		includepath = 1;
 
 	/* Initialize return set */
-	reachable = PySet_New(0);
+	reachable = PySet_New(NULL);
 	if (reachable == NULL) {
 		PyErr_NoMemory();
 		goto bail;