gitk: fix setting font display with new tabbed dialog layout.

The changes to the dialog window tree broke the preview of the selected
font on the button. This corrects that issue.
Signed-off-by: NPat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

gitk

@@ -10795,7 +10795,7 @@ proc fontok {} {
     if {$fontparam(slant) eq "italic"} {
 	lappend fontpref($f) "italic"
     }
-    set w $prefstop.$f
+    set w $prefstop.notebook.fonts.$f
     $w conf -text $fontparam(family) -font $fontpref($f)
 
     fontcan