Restructure HookEngine to use PushLog records for all operations

Summary:
Ref T4195. This pulls the central logic of HookEngine up one level and makes all the git stuff genrate PushLogs.

In future diffs, everything will generate PushLogs and we can hand those off to Herald.

Test Plan:
Pushed a pile of valid/invalid stuff:

{F89256}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7761