We were caching the chunks that made up the get_diff_files list, when the only time the chunks were useful were in get_diff_files. So let's cache the results of that instead. Same with the file coming from the SCM backend, in order to make more room for the get_diff_files cache. In theory, if this falls out of the cache, we'll probably have lost the file contents anyway. Besides, we weren't caching HEAD files anyway.

Caching seems to work in the diff viewer and review request page with comments.