(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.
(diff-minor-mode, diff-minor-mode-prefix, diff-minor-mode-map):
New code to support the minor mode version.
(diff-recenter): New function.
(diff-next-hunk, diff-next-file): Use it.
(diff-remembered-files-alist): New var.
(diff-merge-strings): New function.
(diff-find-file-name): Make it smarter and use the user's input more.
(diff-mode): Cosmetic changes.
