The CVS implementation lacked any unit tests around diffs. This change

adds some tests, testing basic diff functionality, include_files=, and

exclude_patterns=.
Some of the diff logic was reordered to help be more readable, but no

actual behavior has changed.
Note that CVS has its own cvs diff, tool, so we don't directly depend

on GNU diff.
The data for the CVS repository in testdata will be committed in a

separate change.

Unit tests pass on Python 3.7-3.11.