Fix some weird behaviour with diffs

Summary:
1. It was weird when you would click "show unmodified" and a single row
would show up, taking up the exact same amount of space as before.
2. Putting the changed items first turned out to not make as much sense
as I thought - it was really weird when you would have multiple choice
questions where the 3rd answer changed, and the order would be
2, 0, 1, 3

Test Plan:
* Open difftest.html
* See the 'content' item is already expanded since it's the only hidden
  thing
* expand the answers
* they should be in order

Reviewers: alex, joel

Reviewed By: joel

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7493