bug#13907: 24.3.50; cygw32 build mishandles drag-n-dropped file with non-ASCII characters in name

[Eli Zaretskii]

> Date: Fri, 08 Mar 2013 22:33:07 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 13907@debbugs.gnu.org
> 
> And one more question: what is the value of file-name _before_ it is
> passed to cygwin-convert-file-name-from-windows?  Does it perhaps
> already have the U-umlaut replaced by a blank?

I think the problem is on the C level, not on the Lisp level.  Take a
look at w32term.c:construct_drag_n_drop -- it uses ANSI version of
DragQueryFile to get the file name, then decodes it by DECODE_FILE.
But DECODE_FILE uses UTF-8 in the cygw32 build, so this is
inappropriate for decoding file names that come from Windows APIs.

Instead, in the cygw32 build, construct_drag_n_drop should use
DragQueryFileW and convert the file name to the internal Emacs
representation using from_unicode.