Bring "Reverts X" to more general infrastructure and port unit tests

Summary:
Ref T3886. See D8261. This brings the "reverts x" phrase to modern infrastructure. It isn't actually called by the real parser yet, I'm going to do that in one go at the end so I can test everything more easily.

This had unit tests; port most of them forward.

Test Plan: Added and executed unit tests.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3886

Differential Revision: https://secure.phabricator.com/D8262