Fix some warnings about markup engines in Audit/Diffusion inline comments

Summary:
Split from D1921.

  - DifferentialChangesetParser doesn't have this property declared.
  - We weren't providing a markup engine, which caused some warnings.
  - This would cause failures if comment caches weren't present. Currently, they always will be though unless someone has wiped them explicitly in the DB.

Test Plan: Viewed a diff with inline comments, didn't get any warnings in the log.

Reviewers: nh, vrana, btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1925

src/applications/differential/parser/changeset/DifferentialChangesetParser.php

@@ -54,6 +54,7 @@ final class DifferentialChangesetParser {
   private $lineWidth = 80;
   private $isTopLevel;
   private $coverage;
+  private $markupEngine;
 
   const CACHE_VERSION = 4;