Separate repository updates from the pull daemon

Summary:
Ref T4605. Currently, the PullLocal daemon is responsible for two relatively distinct things:

  - scheduling repository updates; and
  - actually updating repositories.

Move the "actually updating" part into a new `bin/repository update` command, which basically runs the pull, discover, refs and mirror commands. This will let the parent process focus on scheduling in a more understandable way and update multiple repositories at once. It also makes it easier to debug and understand update behavior since the non-scheduling pipeline can be run separately.

Test Plan:
  - Ran `update --trace` on SVN, Mercurial and Git repos.
  - Ran PullLocal daemon for a while without issues.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4605

Differential Revision: https://secure.phabricator.com/D8780