Support "changed filename" and "file content" fields for commit content Herald rules

Summary: Ref T4195. Adds support for diff content rules.

Test Plan: Pushed SVN and Git changes through, saw them generate reasonable transcripts. Mercurial still isn't hooked up to this phase.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7791