The DiffFile model is quite simple, just being a data storage mechanism

for information about a file in a diff. This file uses a ton of terrible

names for things, and much of the documentation was just flat-out wrong.

This change converts it to TypeScript and spina, and corrects a bunch of

errors in the documentation about the attributes.
This also converts the DiffFileCollection class, which is extremely

small.

Ran js-tests.