FakeRepos is useful for offline SCM tests but it was hardcoded for depot_tools
requirements. Luckily, it was already modular enought that it merely required to
shuffle code around to split it in two classes to be able to reuse the code
as-is.

BUG=
TEST=

Review URL: http://codereview.chromium.org/6029004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@69845 0039d316-1c4b-4281-b951-d872f2087c98