thumbs.el: convert.exe looks for wrong filename in Windows

[Drew Adams]

I tried to use `C-t a' in dired, after loading thumbs. I use cygwin on
Windows; my shell is bash.

I got this error: convert.exe: unable to open image
`c:/.emacs-thumbs/c:drews-lisp-20ScreenShotsCopy_of_thumb.jpg':
Invalid argument.

Looking in c:/.emacs-thumbs, I see that the filename is
"cdrews-lisp-20ScreenShotsCopy_of_thumb.jpg". That is, there is no
colon in the filename.

Here is more from *Messages*:

Loading thumbs...
Loading image-file...done
Loading time-date...done
Loading thumbs...done
convert.exe: unable to open image
  `c:/.emacs-thumbs/c:drews-lisp-20ScreenShotsCopy_of_thumb.jpg':
  Invalid argument.
thumbs-insert-image: Invalid image specification


In GNU Emacs 22.0.50.2 (i386-mingw-nt5.1.2600)
 of 2005-04-16 on LAPTOP
Distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (3.4)'