This updates the Mercurial diff parser to work directly on the

ParsedDiffFile, rather than the deprecated intermediary parser

dictionary. It also reduces repeated attribute accesses on the parser,

removes some state we used to capture that we don't at all use, and to

bring an unnecessary utility function inline to where it was previously

called.
Along with this, the parser methods have been documented, providing

better insight into what each function is responsible for.

Unit tests pass.