bug#2585: 23.0.90; read-file-name with dialog

[Chong Yidong]

> Whoever did the conversion of read-file-name from C to lisp probably 
> knows what was meant by this, by my naive impression is that the 
> following logic is a mistake:
>
>   ;; If DIR contains a file name, split it.
>   (let ((file (file-name-nondirectory dir)))
>     (when (and default-filename (not (zerop (length file))))
>       (setq default-filename file)
>       (setq dir (file-name-directory dir)))
>
> The third line makes more sense to me as:
>
>  (when (and (not default-filename) (not (zerop (length file))))
>
> ie. if default-filename is specified, then the splitting should NOT be
> done.

That seems right to me.  Stefan, you were the one who rewrote
read-file-name; could you verify this?