The previous version struggled with trailing linebreaks. Fix that by making
sure we ignore any changes on the final newline (Jinja2 doesn't write those
out anyway).

Let's make sure we read, split and rejoin old and new data in exactly the same
way so we don't have to deal with stray line endings.

And the generator returned by difflib is always True. Unclear why this worked
in my local tests when I wrote the original support...

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>