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 PhutilDirectoryFixture-based repositories.

Next steps:

1. Do the same for repo discovery.
2. Then we can start writing tests against specific Conduit methods.

Test Plan: Ran unit tests. Ran bin/repository pull on SVN, Hg and Git repositories. Ran bin/phd debug pulllocal.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, nh

Maniphest Tasks: T2784

Differential Revision: https://secure.phabricator.com/D5904