R1:dbdb01f85807

Fix whitespace and unchanged file shields

Summary:
Fixes T181. I actually have no idea what the original issue in T181 was, but this fixes several problems:

- The code for figuring out whitespace-only changes was extremely confusing and probably buggy (the code for figuring out unchanged files was equally confusing but probably less buggy).
- When rendering a whitespace shield, we did not offer a "Show Changes" link. Instead, show the "Show Changes" link: we can always render the content beneath this…