Perform Git operations without changing CWD.

unittests/package_test.py

 #!/usr/bin/env python
 # Copyright 2015 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.
 
 import doctest
 import os
 import sys
 import unittest
 
@@ skipping 46 matching lines @@
     self.mock_os.path.exists.return_value = False
     self.mock_os.path.isdir.return_value = False
 
     self.repo_spec.checkout(self.context)
 
     self.mock_subprocess.check_call.assert_any_call(
         ['git', 'clone', self.REPO_URL,
          os.path.join(self.context.package_dir, 'funny_recipes')],
          cwd=None)
     self.mock_subprocess.check_call.assert_any_call(
-        ['git', 'reset', '-q', '--hard', 'deadbeef'],
-        cwd='repo/root/recipes/.recipe_deps/funny_recipes')
+        ['git',
+         '--git-dir', 'repo/root/recipes/.recipe_deps/funny_recipes/.git',
+         'reset', '-q', '--hard', 'deadbeef'], cwd=None)
 
 
 class MockProtoFile(package.ProtoFile):
   def __init__(self, path, text):
     super(MockProtoFile, self).__init__(path)
     self._text = text
 
   @property
   def path(self):
     return self._path
@@ skipping 130 matching lines @@
     self.assertEqual(foo_deps.repo_spec.path, '/path/to/local/foo')
 
 
 def load_tests(_loader, tests, _ignore):
   tests.addTests(doctest.DocTestSuite(package))
   return tests
 
 
 if __name__ == '__main__':
   result = unittest.main()