Separate repository mirroring into an Engine and provide `bin/repository mirror`

Summary:
Ref T4338. Currently, there's no diagnostic command to execute mirroring (so I can't give users an easy command to run), and it's roughly the last piece of real logic left in the PullLocal daemon.

Separate mirroring out, and provide `bin/repository mirror`.

Test Plan:
  - Ran `bin/repository mirror` to mirror a repository.
  - Ran PullLocalDaemon and verified it also continued mirroring normally.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4338

Differential Revision: https://secure.phabricator.com/D8066