Trivial fix for publishing with comma-editable fields and None values.

Comma-editable fields were, by default, wrapping the old and new values
in a set() when checking if the values had changed, in order to do
order-independent comparisons.

However, if the value was None, instead of an empty list, they would
error out. A None value may be perfectly legal for that field, so we
need to allow for this.

This change fixes this by converting None to an empty list when building
the sets.
Make get_line_changed_regions accessible in diffutils.

get_line_changed_regions is going to be used outside of ChunkGenerator.
This is a simple change that pulls it out of there and into diffutils,
where it can be accessed as public API.
Switch over to django's bundled six library.

Now that we've moved to Django 1.6, we no longer need to use the copy shipped
in djblets. This changes all the imports.

Testing done: Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/5497/