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.

Viewed a markdown file attachment in both single-revision and diff

views. Verified that switching between "rendered" and "source" worked

properly for both.