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