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

[Ken Brown <kbrown@cornell.edu>]

Create a file "/tmp/Ü.txt".  (In case my mailer mangles this, that's 
<U-umlaut>.txt.)  Now open Windows Explorer and drag this file into an 
emacs frame.  This results in the error message

   dnd-open-local-file: Can not read file:///tmp/%20.txt

I think the problem occurs early in w32-handle-dropped-file (defined in 
lisp/term/w32-win.el).  That function starts with

(let ((f (if (eq system-type 'cygwin)
	     (cygwin-convert-file-name-from-windows file-name t)

At this point f should have the value "/tmp/Ü.txt".  If I continue 
manually carrying out the code in w32-handle-dropped-file as though f 
had that value, everything is fine, as shown below.  So 
cygwin-convert-file-name-from-windows must be doing something wrong.

Here's a session in the *scratch* buffer, imitating what 
w32-handle-dropped-file would do starting from the correct value of f:

(setq f "/tmp/Ü.txt")
"/tmp/Ü.txt"

(setq coding (or file-name-coding-system
		 default-file-name-coding-system))
utf-8-unix

(setq file-name
       (mapconcat 'url-hexify-string
		 (split-string (encode-coding-string f coding)
			       "/")
		 "/"))
"/tmp/%C3%9C.txt"

(setq uri (concat
	   (if (eq system-type 'cygwin)
	       "file://"
	     "file:")
	   file-name))
"file:///tmp/%C3%9C.txt"

(dnd-get-local-file-name uri t)
"/tmp/Ü.txt"


In GNU Emacs 24.3.50.4 (i686-pc-cygwin)
  of 2013-03-06 on fiona
Bzr revision: 111954 dmantipov@yandex.ru-20130306112630-ooq2zc4oq664z5zc
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
  `configure --with-w32 CFLAGS=-g3 -O0'

Important settings:
   value of $LANG: en_US.UTF-8
   locale-coding-system: utf-8-unix
   default enable-multibyte-characters: t