bug#24057: 25.1.50; ffap interprets comments beginning with "//" as file path

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@users.sourceforge.net>
> Date: Sat, 23 Jul 2016 14:02:36 -0400
> Cc: Kaushal Modi <kaushal.modi@gmail.com>, 24057@debbugs.gnu.org
> 
> How does it work?
> 
> C:\Users\npostavs>echo foo > //tmp
> The specified path is invalid.

Oh, and of course you cannot use forward slashes in CMD, you need to
use backslashes instead, as in \\tmp\foo.