Matthias Meulien <orontee@gmail.com> writes:

>> (...)  Could the above hook be replaced with customization of
>> diff-outline-default-state?

With the attached patch, Outline minor mode is automatically enabled
when `diff-outline-default-state' is customized. Tested within "emacs
-q". That said, I am not a proficient elisper and my use of the
autoload, declare-function, eval-when-compile machinery may not be
standard. Sorry for this.