gitk: Limit how often we change the canvas scrolling region

For some unknown reason, changing the scrolling region on the canvases
provokes multiple milliseconds worth of computation in the X server,
and this can end up slowing gitk down significantly.  This works around
the problem by limiting the rate at which we update the scrolling region
after the first 100 rows to at most 2 per second.
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -2616,6 +2616,7 @@ proc layoutmore {tmax allread} {
 proc showstuff {canshow last} {
     global numcommits commitrow pending_select selectedline curview
     global lookingforhead mainheadid displayorder nullid selectfirst
+    global lastscrollset
 
     if {$numcommits == 0} {
 	global phase
@@ -2623,8 +2624,13 @@ proc showstuff {canshow last} {
 	allcanvs delete all
     }
     set r0 $numcommits
+    set prev $numcommits
     set numcommits $canshow
-    setcanvscroll
+    set t [clock clicks -milliseconds]
+    if {$prev < 100 || $last || $t - $lastscrollset > 500} {
+	set lastscrollset $t
+	setcanvscroll
+    }
     set rows [visiblerows]
     set r1 [lindex $rows 1]
     if {$r1 >= $canshow} {