Previously diff cursors initialized when their diffs completely finished
rendering. This means waiting longer than needed because the diff render
process includes processing and rendering syntax highlights whereas the
diff cursor only needs to wait on the content of the diff.
With this change diffs fire an event when they've finished with their
content only, and diff cursors await this event instead of the full
render.
Bug: Issue 5681
Change-Id: Ida9da83a20fe4c2dcd8920304504ec7e1185eb5d