Don't call `updateDiff()` in the Revisions FrameState model.

* Stuff needs to hook in, so firing it early results in a half-rendered screen.
* `updateDiff()` now returns a promise.
* Now, in the frame view's `render()`, it calls `updateDiff()`, relying on its promise.

See #24425.

git-svn-id: https://core.svn.wordpress.org/trunk@24622 1a063a9b-81f0-0310-95a4-ce76da25c4cd