Add a DOM function for generating the "stats" metadata.
Review Request #11743 — Created July 23, 2021 and submitted
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.
F841 local variable 'meta_stats' is assigned to but never used