Modernize and document the Perforce diff parser.
Review Request #10782 — Created Nov. 4, 2019 and submitted
This updates the Perforce diff parser to work directly on the
ParsedDiffFile, rather than the deprecated intermediary parser
dictionary. It also reduces repeated attribute accesses on the parser.
Along with this, the parser methods have been documented, providing
better insight into what each function is responsible for.
Unit tests pass.