Unfatal the "show change" link in Maniphest description edits

Summary: I missed this callsite in D2946. Transition it to the new markup cache.

Test Plan: Clicked "show change" on a description edit transaction, got the change instead of a fatal.

Reviewers: alanh, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1502

Differential Revision: https://secure.phabricator.com/D2972