gitk: Fix bug where the last few commits would sometimes not be visible

We weren't calling showstuff for the last few commits under some
circumstances, causing the scrolling region not to be extended right
to the end of the graph.  This fixes it.

Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -2758,7 +2758,7 @@ proc layoutmore {} {
     global uparrowlen downarrowlen mingaplen curview
 
     set show $commitidx($curview)
-    if {$show > $numcommits} {
+    if {$show > $numcommits || $viewcomplete($curview)} {
 	showstuff $show $viewcomplete($curview)
     }
 }