gitk: Fix bug where page-up/down wouldn't always work properly

If the user pressed page up or page down and the new page wasn't
already drawn, we failed to select the line we wanted in the new
page.  This fixes it.

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

gitk

@@ -3429,6 +3429,7 @@ proc selnextpage {dir} {
 	set lpp 1
     }
     allcanvs yview scroll [expr {$dir * $lpp}] units
+    drawvisible
     if {![info exists selectedline]} return
     set l [expr {$selectedline + $dir * $lpp}]
     if {$l < 0} {