git-gui: Attach font_ui to all spinbox widgets

Earlier I missed making sure our spinbox widgets used the same font
as the other widgets around them.  This meant that using a main font
with a size of 20 would make every widget in the options dialog huge,
but the spinboxes would be left with whatever the OS native font is.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -1205,7 +1205,7 @@ font create font_diffbold
 
 foreach class {Button Checkbutton Entry Label
 		Labelframe Listbox Menu Message
-		Radiobutton Text} {
+		Radiobutton Spinbox Text} {
 	option add *$class.font font_ui
 }
 unset class