bug#20943: 25.0.50; Dired buffers are not always auto-reverterd

[Mark Karpov <markkarpov@openmailbox.org>]

> Can you show a recipe for reproducing this?

Sure.

1. emacs -Q

2. Evaluate the following:

(setq
 dired-dwim-target      t        ; guess target directory
 dired-listing-switches "-GAlh --group-directories-first")

3. Open two Dired buffers side-by-side.

4. Mark some files in the first buffer with ‘m’ key.

5. Press ‘C’ and confirm copying.

6. Files will be copied into another buffer. It's best if the buffer has
   some directories in it. My ‘dired-listing-switches’ tell ‘ls’ to list
   directories first. But chances are newly inserted files won't respect
   this. Thus we get situation that I've described in my previous
   message. For the sake of clarity press ‘g’ now and see how files are
   reordered. Moral here is that you generally cannot (I may be mistaken
   here, this is just my opinion) reliably guess true order of files
   without regeneration of Dired buffer. Well, I don't really know how
   it works in details, you could analyze ‘dired-listing-switches’ and
   try to emulate it, but is it worth the trouble?

I should note that concept of modified buffer is not really applicable
to Dired buffer, because it usually cannot contain anything that doesn't
exists in directory in your system. It's especially true if auto-revert
will work for modified buffers. Any time Dired buffer gets modified it
will be reverted by subsequent invocation of ‘auto-revert-handler’, thus
this state won't last longer than ‘auto-revert-interval’. I don't think
it's bad, but I think it's better to mention this consideration here.