Adding support for 'adds' and 'removes' in diff content.

Summary:
Does what it says on the label. We already had 'Any changed file content', now we have 'Any added file content' and 'Any removed file content'.

• There is a bit of copied/pasted code here: I'm open to suggestions on how to refactor it so it's less redundant.
• The wording seems a little awkward, and as @epriestley mentioned in T3829, moved code will be detected less than ideally.

Test Plan: Created Herald Rules, verified via dry run that they were triggered in appropriate situations.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T3829

Differential Revision: https://secure.phabricator.com/D7214