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