difftool--helper: fix printf usage

Do not use a random string as if it is a format string for printf
when showing it literally; instead feed it to '%s' format.

Reported-by: Asheesh Laroia <asheesh@asheesh.org>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>