This fixes a bug with the show content link for deleted files in the diff

viewer. The link wouldn't show the content of the file and would instead

remove it from the diff viewer. This is fixed by properly replacing existing

file diffs in the diff viewer when the show content link is clicked.

Clicked on show content for multiple files in a diff viewer and saw that

  they were shown properly.
Saw that the diff file index properly listed files depending on which

  files are visible on screen, both before and after clicking show content.
Saw that diffs were properly rendered for non-deleted files.
Ran JS unit tests.