git-gui: Set both 16x16 and 32x32 icons on X to pacify Xming.

It would be better if the 32x32 icon was equivalent to the one used on
Windows (in git-gui.ico), but I'm not sure how that would best be done,
so I copied this code from gitk instead.

Signed-off-by: Samuel Bronson <naesten@gmail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

git-gui.sh

@@ -729,7 +729,10 @@ if {[is_Windows]} {
 		gitlogo put gray26  -to  5 15 11 16
 		gitlogo redither
 
-		wm iconphoto . -default gitlogo
+		image create photo gitlogo32 -width 32 -height 32
+		gitlogo32 copy gitlogo -zoom 2 2
+
+		wm iconphoto . -default gitlogo gitlogo32
 	}
 }