The view mode selector ("rendered" vs "source") wasn't working when viewing a diff of a text-based file attachment. We initially noticed this with the Markdown review UI, but it affects all text-based reviewables that support render modes. The issue here is that we were passing in a `root` to `Backbone.history` that wasn't properly including the diff file attachment ID. Because the root didn't match, the route would never get triggered when the URL changed after the anchor was clicked. Testing Done: Viewed a markdown file attachment in both single-revision and diff views. Verified that switching between "rendered" and "source" worked properly for both.