This introduces DiffX.generate_stats(),

DiffXChangeSection.generate_stats(), and

DiffXFileSection.generate_stats(). These are used to generate metadata

containing useful statistics on the diff, such as the number of files,

changes, inserts/deletes, and changed lines in a diff.
Callers can call this immediately prior to writing the DiffX to a

stream, in order to provide this information for consumers of the diff.

Unit tests pass on Python 2 and 3.