difftool: Update copyright notices to list each year separately

This is http://www.gnu.org/licenses/gpl-howto.html advises.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git-difftool--helper.sh b/git-difftool--helper.sh
index 69f6bce..e43b5d6 100755
--- a/git-difftool--helper.sh
+++ b/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