Fix bad interpreters.

Somehow some (but not all) git helper scripts got the wrong interpreter. This
fixes them to use /usr/bin/env.

TBR=agable@chromium.org
BUG=

Review URL: https://codereview.chromium.org/212293002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@259522 0039d316-1c4b-4281-b951-d872f2087c98

git_freezer.py

-#!/usr/local/bin/python
+#!/usr/bin/env python
 # Copyright 2014 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.

git_new_branch.py

-#!/usr/local/bin/python
+#!/usr/bin/env python
 # Copyright 2014 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.

git_rebase_update.py

-#!/usr/bin/python
+#!/usr/bin/env python
 # Copyright 2014 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.