[commit-patch-buffer.el] no need to load diff-mode, just name sure our hook gets added if it ever does load

Hi David,

our developers were interested in using commit-patch to help with their
code commits to our subversion repository. I've created a patch that
adds support for subversion. We've been using this internally and
thought that you might be interested in the code addition. I've attached
the patch to this e-mail (gzipped to avoid issues with mail scanners).

Cheers,
Peter

(It doesn't strip formatting out of the name when it creates the
title)

This command does the diff for you, launches your editor and lets you edit the diff and the
commit comment in one file. Inspired by Pistos <pistos@gmail.com>.