Not all chunks were getting an index attribute assigned, which lead to many tbody elements in the diff view having the same id attribute (e.g, "chunk0."). Now all chunks are indexed.

Also removed some unnecessary work from diffutils.add_navigation_cues while I was at it.

Verified that all chunks in a given patch now have unique id attributes. Verified that the change index at the top and bottom of the diff still functions correctly. Verified that only changed chunks are selected when using keyboard shortcuts such as j/k or n/p.