revlog: fix excessive decref on tuple creation failure in parse_index2()

Since Py_BuildValue() steals the ownership of "N" arguments, these objects
would already be freed if Py_BuildValue() returned NULL.

https://github.com/python/cpython/blob/2.7/Python/modsupport.c#L292

mercurial/cext/revlog.c

@@ -2885,7 +2885,7 @@
  */
 PyObject *parse_index2(PyObject *self, PyObject *args)
 {
-	PyObject *tuple = NULL, *cache = NULL;
+	PyObject *cache = NULL;
 	indexObject *idx;
 	int ret;
 
@@ -2906,11 +2906,8 @@
 		Py_INCREF(cache);
 	}
 
-	tuple = Py_BuildValue("NN", idx, cache);
-	if (!tuple)
-		goto bail;
-	return tuple;
+	return Py_BuildValue("NN", idx, cache);
 
 bail:
 	Py_XDECREF(idx);
 	Py_XDECREF(cache);
@@ -2913,8 +2910,7 @@
 
 bail:
 	Py_XDECREF(idx);
 	Py_XDECREF(cache);
-	Py_XDECREF(tuple);
 	return NULL;
 }