show whitespace problems by default in diff-mode

[Dan Nicolaescu]

diff-mode is set up to show trailing whitespace problems, but it's not
doing it by default, to enable this one has to do:

(add-hook 'diff-mode-hook 'whitespace-mode)

Vinicius also posted a patch to edit files to clean the trailing
whitespace problems based on the diff.

Why not turn on whitespace-mode by default in diff-mode?  It makes
spotting whitespace problems very easy, and will make it also easy to
fix.