reachableroots: if allocating a new set fails, use PyErr_NoMemory()

My inspection of the implementation of PySet_New() indicates that it
does *not* reliably set an exception in the cases where it returns
NULL (as far as I can tell it'll never do that!), so let's set that up
ourselves.

mercurial/parsers.c

@@ -1143,8 +1143,10 b' static PyObject *reachableroots(indexObj'
 
 	/* Initialize return set */
 	reachable = PySet_New(0);
-	if (reachable == NULL)
+	if (reachable == NULL) {
+		PyErr_NoMemory();
 		goto bail;
+	}
 
 	/* Initialize internal datastructures */
 	tovisit = (int *)malloc((len + 1) * sizeof(int));