gitk: Fix a bug in collapsing deeply nested trees

To reproduce: expand a tree like this, then collapse A:

+A
 +B
   C
   D

The result is:

-A
   C
   D

I.e. sub-nodes expanded from the last sub-node of the item
being collapsed are not removed.
Signed-off-by: NAlexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -2706,7 +2706,7 @@ proc treeopendir {w dir} {
 	    $w insert e:$ix $e [highlight_tag $de]
 	}
     }
-    $w mark gravity e:$ix left
+    $w mark gravity e:$ix right
     $w conf -state disabled
     set treediropen($dir) 1
     set top [lindex [split [$w index @0,0] .] 0]