refactor pass 2 of N on DifferentialChangesetParser and bonus bug fix

Summary: pull out some more stuff from the TwoUp renderer that's generically useful. also clean up $xhp variable and add a get method for the $coverage. Finally, fix T2177 while I'm in here.

Test Plan: played around with Differential. Also opened things up in Firefox to verify T2177.

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2009, T2177

Differential Revision: https://secure.phabricator.com/D4161