More convenient default rollback for users

[Lars Magne Ingebrigtsen]

When we introduce behavioural changes in Emacs, we often include a way
to get back to the old behaviour, and we document this in NEWS etc.

But wouldn't it be nice if we had this in a more convenient,
cut-and-pastable format?

Imagine you're using Emacs 26 for the first time.  Suddenly you notice
that when you hit the `hyper z' key, Emacs doesn't florbnoze as it used
to, but forblizes instead!  You're furious!  So you hit the `C-h R' (for
rollback) keystroke, and you get a buffer that looks like:

-------------
;; To roll back changes between Emacs 26 and Emacs 25.1:

;; If you want the `hyper z' key still to florbnoze instead of
;; forblizing:
(setq hyper-z-mode 'florbnoze)

... more setqs here...

;; To roll back changes between Emacs 25.1 and Emacs 24.3:

...
-------------

And then the anger dissipates.  No need to read documentation or
anything.  Just get back the old behaviour you enjoyed.

(And this would also mean that we could introduce new behaviours more
often, because rollback would be so trivial for the users.)

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no