Slightly modernize transaction diff controller
Summary: Ref T9272. This doesn't fix anything, just a little cleanup while I was looking at it.
Test Plan: Clicked "Show Details" on a couple description changes, got the same effect for less code.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T9272
Differential Revision: https://secure.phabricator.com/D14168