gitk: Avoid Tcl error when switching views

Michele Ballabio <barra_cuda@katamail.com> pointed out that gitk
sometimes throws a Tcl error (can't read "yscreen") when switching
views, and proposed a patch.  This is a different way of fixing it
which is a bit neater.  Basically, in showview we only set yscreen if
the selected commit is on screen to start with, and then we only
scroll the canvas to bring it onscreen if yscreen is set and the
same commit exists in the new view.

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

gitk

@@ -2102,8 +2102,6 @@ proc showview {n} {
 	set ybot [expr {[lindex $span 1] * $ymax}]
 	if {$ytop < $y && $y < $ybot} {
 	    set yscreen [expr {$y - $ytop}]
-	} else {
-	    set yscreen [expr {($ybot - $ytop) / 2}]
 	}
     } elseif {[info exists pending_select]} {
 	set selid $pending_select
@@ -2164,7 +2162,7 @@ proc showview {n} {
     set yf 0
     set row {}
     set selectfirst 0
-    if {$selid ne {} && [info exists commitrow($n,$selid)]} {
+    if {[info exists yscreen] && [info exists commitrow($n,$selid)]} {
 	set row $commitrow($n,$selid)
 	# try to get the selected row in the same position on the screen
 	set ymax [lindex [$canv cget -scrollregion] 3]