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:

  # Do the same for repo discovery.
  # 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