[Findit] Factoring the gitiles (etc) stuff out into its own directory

appengine/findit/common/test/chromium_deps_test.py

Index: appengine/findit/common/test/chromium_deps_test.py
diff --git a/appengine/findit/common/test/chromium_deps_test.py b/appengine/findit/common/test/chromium_deps_test.py
index c217f8fbdb5251e37746fcdfed1ef7cdc27d9bac..f6f41627c67608491b5e7bac00a612d43c96b144 100644
--- a/appengine/findit/common/test/chromium_deps_test.py
+++ b/appengine/findit/common/test/chromium_deps_test.py
@@ -7,16 +7,24 @@ import collections
 
 from testing_utils import testing
 
+from gitiles import git_repository
 from common import chromium_deps
 from common import deps_parser
-from common import git_repository
-from common import repository
 from common import retry_http_client
 from common import http_client_appengine
 from common.dependency import Dependency, DependencyRoll
 
 
-class DummyGitRepository(repository.Repository):
+class DummyGitRepository(object):
+  """A class for mocking GitRepository.
+
+  N.B., in order to use this class for mocking, every module we want to
+  test with a mock GitRepository must not import that class directly.
+  Instead they must import the git_repository module and rely on dynamic
+  dispatch to resolve the GitRepository attribute. Otherwise those
+  modules will hold direct links to the real GitRepository class, and
+  so won't dispatch into this mock class for our unit tests."""
+
   RESPONSES = {}
 
   def __init__(self, *_):