Teach mergetool to get the list of files to edit via `diff` so that we
gain support for diff.orderFile.
Suggested-by: Luis Gutierrez <luisgutz@gmail.com> Helped-by: Johannes Sixt <j6t@kdbg.org> Signed-off-by: David Aguilar <davvid@gmail.com> Reviewed-by: Johannes Sixt <j6t@kdbg.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>