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

src/applications/repository/engine/PhabricatorRepositoryEngine.php

@@ -47,6 +47,11 @@ abstract class PhabricatorRepositoryEngine {
   }
 
 
+  public function getViewer() {
+    return PhabricatorUser::getOmnipotentUser();
+  }
+
+
   /**
    * @task internal
    */