git-gui: fix usage of themed widgets variable

There was one forgotten global so NS was not visible to the method
which resulted in an error.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>

git-gui/lib/status_bar.tcl

@@ -39,6 +39,7 @@ method _oneline_pack {} {
 }
 
 constructor two_line {path} {
+	global NS
 	set w $path
 	set w_l $w.l
 	set w_c $w.c