(diff-kill-junk): New interactive function.

(diff-kill-junk): New interactive function.
(diff-reverse-direction): Use delete-and-extract-region.
(diff-post-command-hook): Restrict the area so that the hook also works
outside of any diff hunk.  This is necessary for the minor-mode.
(diff-mode): Use toggle-read-only and minor-mode-overriding-map-alist.
(diff-minor-mode): Setup the hooks for header-hunk rewriting.