gitk: Fix incorrect invocation of getmergediffline

Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/gitk b/gitk
index e1848cd..f88c06e 100755
--- a/gitk
+++ b/gitk
@@ -2700,7 +2700,7 @@
 	incr nextupdate 100
 	fileevent $mdf readable {}
 	update
-	fileevent $mdf readable [list getmergediffline $mdf $id]
+	fileevent $mdf readable [list getmergediffline $mdf $id $np]
     }
 }