Support rich diff rendering with DocumentEngine for added/removed files

Summary: Ref T13425. When a file (like a Jupyter notebook) is added or removed, we can still render a useful line-by-line diff.

Test Plan:

• Viewed add/modify/remove of Jupyter, source code, and images in 2up/1up mode, everything looked okay.

Maniphest Tasks: T13425

Differential Revision: https://secure.phabricator.com/D20865