I am not sure these patches should be applied as-is.  Some things I
wonder about:

1. About read--propertize-prompt…

    1. Should the function return a copy of its argument instead of
       propertizing it directly?

    2. Is it properly named?  Does it fit in subr.el?  I placed it there
       because I figured other users of read-char in subr.el could use
       it, e.g. read-char-choice.

2. dired-aux.el already contains some logic to detect isolated
   characters; I could not think of a way to re-use it, so I added my
   own functions to find *non*-isolated characters.  I added unit tests
   for these new functions; still, there may be some redundancy there.

WDYT?


PS1: I am still absolutely open to simply rephrasing the prompt…  I just
cannot come up with good alternatives.

PS2: CC'ing Stefan to follow up on the discussion on emacs-devel.
<https://lists.gnu.org/archive/html/emacs-devel/2019-05/msg00339.html>