git-gui: fix "Explore Working Copy" for Windows again

It has already been fixed in commit 454efb47 (git-gui (Win): make
"Explore Working Copy" more robust, 2009-04-01), but has been broken in
commit 21985a11 (git-gui: handle non-standard worktree locations,
2010-01-23) by accidentally replacing too much with a new variable.

The problem can be reproduced when starting git-gui from within a
subdirectory. The solution is to convert the path name, explorer.exe is
invoked with, to a platform native name.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>

git-gui/git-gui.sh

@@ -2107,7 +2107,7 @@ proc do_explore {} {
 		# freedesktop.org-conforming system is our best shot
 		set explorer "xdg-open"
 	}
-	eval exec $explorer $_gitworktree &
+	eval exec $explorer [list [file nativename $_gitworktree]] &
 }
 
 set is_quitting 0