R1:caa6fdf56dc2

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…