difftool: Update copyright notices to list each year separately

This is http://www.gnu.org/licenses/gpl-howto.html advises.
Signed-off-by: NDavid Aguilar <davvid@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-difftool--helper.sh

@@ -3,7 +3,7 @@
 # This script is typically launched by using the 'git difftool'
 # convenience command.
 #
-# Copyright (c) 2009-2010 David Aguilar
+# Copyright (c) 2009, 2010 David Aguilar
 
 TOOL_MODE=diff
 . git-mergetool--lib

t/t7800-difftool.sh

 #!/bin/sh
 #
-# Copyright (c) 2009 David Aguilar
+# Copyright (c) 2009, 2010 David Aguilar
 #
 
 test_description='git-difftool