bug#21598: 24.5; Auto-revert-mode loads images as text

[Eli Zaretskii]

> Date: Thu, 01 Oct 2015 22:01:57 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 21598@debbugs.gnu.org
> 
> > From: Matthias Redies <rediesmatthias@yahoo.de>
> > Date: Thu, 1 Oct 2015 17:17:03 +0200
> > 
> > 1. open image C-x C-f and select "output.png". This will display the 
> > image as an image as it should.
> > 2. change Image using an external program. This causes the
> > auto-revert-mode to reload the image(good), but as a text, although my
> > default (as in 1.) is to open the picture as an image
> 
> This happens because autorevert.el calls revert-buffer with its 3rd
> argument non-nil.  It is easy to change that to nil for buffers under
> image-mode, but I'm curious why doesn't autorevert.el do that for all
> the other modes.  Does anyone know?

Actually, I see that image-mode installs an after-revert-hook, bit the
hook function, image-after-revert-hook, does nothing in this case
because image-get-display-property returns nil.