Fix fatal in Maniphest when viewing "show details" of a task description edit

Summary:
Clicking "show details" of a task description change in Maniphest currently throws an exception about the markup engine.

Since we don't actually need the engine an alternate fix would be "if ($this->markupEngine) { $renderer->setMarkupEngine($this->markupEngine); }" but we have one at the ready so just provide it. This should become part of the Transactions stuff anyway.

Test Plan: Clicked "show details".

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4167