bug#10489: 24.0.92; dired-do-copy may create infinite directory hierarchy

[Eli Zaretskii]

> From: Thierry Volpiatto <thierry.volpiatto@gmail.com>
> Cc: 10489@debbugs.gnu.org
> Date: Fri, 24 Feb 2012 13:18:05 +0100
> 
> Have modified `file-subdir-of-p' according to your advices.
> Please have a look. (Tested with success on windows also)

Looks good to me, thanks.