R1:105dd1899c35

Make repository pulls testable

Summary:
Ref T2784. This moves us toward being able to test the background and Conduit pipelines for repositories. In particular:

- Separate the logic for pulling repositories (`git pull`, `hg pull`) out of `PhabricatorRepositoryPullLocalDaemon` and put it in `PhabricatorRepositoryPullEngine`. This allows repositories to be pulled directly without invoking the daemons.
- Add tests for the engine, including a future-looking base test case.
- Add basic…