bug#22025: Emacs 25 corrupts Emacs 24 .emacs.desktop.

[Alan Mackenzie]

Hello, Emacs

Having recently saved a desktop in Emacs 24.5, run Emacs 25, allowing
desktop to load all ones files.  Exit Emacs 25.

Start Emacs 24.5 again.  The desktop fails to load.  This is a Bad
Thing.

Diagnosis
---------

Emacs 25 has read a version 206 .desktop file, but written a version 208
file on termination.  Emacs 24 cannot execute this version.

There was no yes-or-no-p asking "are you sure you want to convert your
desktop to a format incompatible with previous Emacs versions?".

Manually restoring a .desktop to version 206 is fraught with
difficulties, even for somebody (myself) who understands the issues
reasonably well, and has an old version of .desktop saved "just in
case".

Recommendation
--------------

Emacs 25, having loaded a version 206 .desktop, should save it in the
same format until the user has positively consented to it being
converted to version 208.

-- 
Alan Mackenzie (Nuremberg, Germany).