bug#66061: 30.0.50; [PATCH] diff-buffer-with-file should reverse the order if the file is modified

[Bob Rogers]

   From: Eli Zaretskii <eliz@gnu.org>
   Date: Mon, 18 Sep 2023 13:47:39 +0300

   > From: Bob Rogers <rogers@rgrjr.com>
   > Date: Sun, 17 Sep 2023 16:54:41 -0700
   > 
   > . . .
   >
   > The attached patch reverses the order of the arguments to diff only
   > when the buffer is unmodified.  Either the file has changed on disk, in
   > which case it is probably newer, or it hasn't, in which case the diff
   > will be empty anyway so the order doesn't matter (and that saves
   > checking the actual file mod time).

   I don't think we can automatically always reverse them in this case.
   Here's a simple case where we shouldn't:

     . visit a file
     . make some edits
     . save the buffer to the file
     . copy from the backup file (or some other previous version) over
       the edited file on disk

I thought of that, but figured this was a much less likely scenario, and
could be ignored for the sake of getting closer to the right thing.

   I think only the user knows what is "old" and what is "new".  We
   should, of course, allow the user to reverse them, but we shouldn't
   reverse automatically.

Fair enough.  I suppose M-x diff-reverse-direction is not that much
extra trouble.  (I should probably find a keybinding for it.)

                                        -- Bob