Add a basic push log for recording repository push events

Summary:
Ref T4195. This log serves two purposes:

  - It's a log, so you can see what happened. Particularly, in Git/Hg, there is no other way to tell:
    - Who //pushed// a change (vs committed / authored)?
    - When was a change pushed?
    - What was the old value of some tag/branch before someone destroyed it?
  - We can hand these objects off to Herald to implement pre-commit rules.

This is a very basic implementation, but gets some data written and has a basic UI for it.

Test Plan: {F87339}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7705