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