bug#51040: No curved quotes in format-prompt and minibuffer-default-prompt-format

[Stefan Kangas]

It seems like `format-prompt' and `minibuffer-default-prompt-format'
does not support curved quotes.  Should they?

To reproduce, evaluate this forms in emacs -Q:

    (format-prompt "foo `bar'" nil)

    => "foo `bar': "

And:

    (let ((minibuffer-default-prompt-format " (default `%s')"))
      (format-prompt "foo `bar'" t))

    => "foo `bar' (default `t'): "

My expectation is that I get this, where fancy quotes are supported:

    "foo ‘bar’ (default ‘t’): "

Now, this is easy to fix, but we need to decide if it's okay that
`format-prompt' just runs `substitute-command-keys' on these by default.
I believe that fix should be fine, but maybe I'm missing something.