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