Diff parsing is one of the oldest parts of Review Board, and has shown

its age. It used to be that DiffParser and its subclasses would

populate a dictionary with keys and values. Eventually, we introduced a

File class (later renamed ParsedDiffFile) that would hold the

resulting information, but only after the dictionaries were populated.

This approach was error-prone (partly fixed in 4.0 when validation was

added), and made it hard to expand the type of information we collect.
This change improves the capabilities of parsers by giving them direct

access to a ParsedDiffFile object, removing the intermediary

dictionary. It also documents each of the methods in detail, discussing

each one's responsibilities and what subclasses may want to do to

customize behavior.
Currently, only the base DiffParser uses the new way of setting

attributes, to ensure that backwards-compatibility works as expected.

Other parsers will be updated shortly.

Unit tests pass in both Review Board and Power Pack.