The Gitiles code is messy and hard to follow; we should clean it up. In particular:

* Move all the Gitiles stuff to its own directory (not in ./common along with a bunch of unrelated stuff)

* Get rid of the useless Repository class (the class does nothing, and we currently have no intentions of supporting non-git repos). Looks like the only reason it's there is so the unit tests can create a dummy repo for mock tests. Surely there's a cleaner way to mock, which better respects the actual API of the GitRepository class.

* Ideally we should be able to define a basic version of GitRepository which doesn't do the caching stuff (since that's appengine-specific), but which can be easily extended with a subclass that does.