bug#68815: Unexpected behavior with read-file-name and functional REQUIRE-MATCH argument

[Stefan Monnier]

> emacs -Q on the tip of the emacs-29 branch (after commit 77f5d4d523a), run...
>
> (let ((default-directory "~/"))
>   (read-directory-name "Clone into new or empty directory: " nil nil
>                        (lambda (dir) (or (not (file-exists-p dir))
>                                     (directory-empty-p dir)))))
>
> ...then type "/tmp/" (the whole minibuffer now reads "~//tmp") then RET.
>
> Expected: Completion does not exit, instead saying "[No match]".

Ah, that good old problem about whether PRED should apply to the quoted
or to the unquoted string 🙁

I think here the bug is in `read-directory-name` (and `read-file-name`)
since their PRED arg is expected to apply to the (unquoted) file name
itself (i.e. the thing that would be returned by the function), rather
than to the quoted representation used in the minibuffer.

They should wrap PRED so as to pass the arg through
`substitute-in-file-name` (or otherwise arrange to make sure PRED is
called with an unquoted file name).


        Stefan