A recent change improved performance in the diff viewer by optimizing

selectors. One of the selectors optimized was the one used to find

anchors in a diff. Unfortunately, it only looked in <tbody> tags, and

not in <thead>, skipping the file anchors. This prevented these

anchors from navigating to file headers, and for review requests

containing newly-introduced files, it wasn't possible to navigate to the

diffs.
This change fixes the selector and adds a new suite of tests to ensure

that anchor loading, navigation, and key bindings all work correctly.

Tested manually that diff navigation worked for all diffs I threw at it.
Unit tests pass.