parsers: handle refcounting of "parents" consistently

Py_None can be refcounted like any other Python object. So
do that.

mercurial/parsers.c

@@ -2739,6 +2739,7 b' static PyObject *fm1readmarker(const cha'
 		data += nparents * hashwidth;
 	} else {
 		parents = Py_None;
+		Py_INCREF(parents);
 	}
 
 	if (data + 2 * nmetadata > dataend) {
@@ -2781,8 +2782,7 b' bail:'
 	Py_XDECREF(prec);
 	Py_XDECREF(succs);
 	Py_XDECREF(metadata);
-	if (parents != Py_None)
-		Py_XDECREF(parents);
+	Py_XDECREF(parents);
 	return ret;
 }