Modernize and document the CVS diff parser.
Modernize and document the CVS diff parser.

This updates the CVS diff parser to work directly on the
`ParsedDiffFile`, rather than the deprecated intermediary parser
dictionary. It also reduces repeated attribute accesses on the parser.
This cleans up the code a decent amount.

Along with this, the parser methods have been documented, providing
better insight into what each function is responsible for.

Testing Done:
Unit tests pass.

Reviewed at https://reviews.reviewboard.org/r/10780/

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