Record new commits in the push log

Summary:
Ref T4195. Like the previous diffs, these both create a useful log and give us an object to hand off to Herald.

Surface this information in Diffusion, too, and clean things up a little bit.

Test Plan: {F87565}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7718