Fix issue where diffs fail to update when moving between revisions

Review Request #6261 — Created Aug. 24, 2014 and discarded — Latest diff uploaded

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master
1837d7d...

Reviewers

Backbone.View's this.el is not getting initialized (perhaps intentionally, I cannot tell). The fix is to test for validity before using it.

Checked that loading pages with multiple revisions can be changed with slider in the process of producing diffs. Ran all unit tests.