diff-yank-handler is evil

[Jesper Harder]

In GNU Emacs 21.3.50.18 (i686-pc-linux-gnu, GTK+ Version 2.0.2)
 of 2004-03-23 on defun.localdomain

I wanted to paste a diff to an email.  To my great surprise the diff
was corrupted whenever I yanked it.  I only figured out the cause
after doing `C-u C-=' in the diff buffer.

I think this new feature is bad: it's a big violation of the principle
of least surprise -- I don't expect `yank' to munge the text in
various ways.