bug#38457: 27.0.50; dabbrev-expand regression due to message change

[Eli Zaretskii <eliz@gnu.org>]

Here's one more problem with the new behavior of 'message':

  emacs -Q

  Evaluate:

  (defun my-mesage ()
    (interactive)
    (message "my most important message"))
  (global-set-key [f5] 'my-mesage)
  (setq debug-on-message "important")

  Press F5, observe the debugger pop up, as expected.
  Type C-] to exit the debugger.
  Type M-x, then press F5 => the debugger doesn't start, although the
    message appears that should have triggered the debugger.

The problem with dabbrev-expand and Tramp is similar, in that it stems
from the fact that 'message' leaves the text in the echo area until
the next keypress, whereas 'minibuffer-message' actively removes the
message before returning (instead of letting someone else remove it
when needed).

I think the more general underlying problem here is that
'minibuffer-message' behaves very differently from 'message'.  What we
need is to keep the original behavior of 'message', but teach it not
to overwrite the minibuffer contents under certain conditions.