bug#24387: 25.1.50; w32-convert-standard-filenames no longer works

[Richard Copley <rcopley@gmail.com>]

Hi Eli,

`convert-standard-filename' on Windows was changed in this commit:

commit 438b98e5b70c55129453870b870a139c4a4a77fd
Author: Eli Zaretskii <eliz@gnu.org>
Date:   Mon Apr 25 11:50:37 2016 +0300

    Don't mirror slashes in convert-standard-filename on MS-Windows

    * lisp/w32-fns.el (w32-convert-standard-filename): Don't mirror
    slashes into backslashes.  This avoids producing ugly file names,
    and is deemed no longer necessary, and should certainly be
    unrelated to which shell is in use.

I for one /did/ use filenames created by this function in shell
commands. It was explicitly documented that it would work,
and it still is (see the docstring for `convert-standard-filename').
Now I need to take time out to roll my own version and review
all my callers (and, because I'm a nice person and like to help,
file a bug report and expose myself to the ensuing derision :) ).

Please will you change it back? If not, when that docstring gets
fixed, can it document what people should use instead?