R1:239eca13d753

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".

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