Simplify context parsing and add test coverage

Summary:
  - Remove 'missingNew', etc. It's impossible for a diff to popluate these, as far as I can tell (I can't generate such a diff, or find any which generate it).
  - Add unit tests.

Test Plan: Unit tests, viewed a diff with some missing context.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D4393