Explicitly run 'git cl' commands with python2

b/162537710

Change-Id: I6c4d1da0f41833340b3dd76a48d770aa59a24504
diff --git a/git_cl.py b/git_cl.py
index e3ca0b9..effcac9 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.