bug#42865: 28.0.50; Add new 'copy-region-quietly' defcustom

[Juri Linkov]

>> The message text changes not often.  But the symbol approach is much worse
>> because it takes the freedom from users - in case of symbols the developers
>> decide whether to allow the users to inhibit messages or not.  When 
>> developers
>> allow to inhibit some messages by adding a new symbol to the message 
>> function,
>> then it takes many years until the users can start using new symbols to
>> inhibit messages after the next release.
>
> I think you convinced me, thanks.  I still think people will shoot
> themselves in the foot, but OTOH we are hardly strangers to that.

In bug#44629 Eli reminded about set-message-function, and indeed
it's possible to make it safer by checking both the command name
and message regexp.  For example:

  (put 'kill-ring-save 'inhibit-message "^Copied text ")
  (put 'save-buffer 'inhibit-message "^Wrote ")
  (put 'read-only-mode 'inhibit-message "^View mode: type ")

  (defun inhibit-message-function (message)
    (and this-command
         (get this-command 'inhibit-message)
         (string-match-p (get this-command 'inhibit-message) message)))

  (setq set-message-function 'inhibit-message-function)