git-am: error out when seeing -b/--binary

The --binary option to git-apply has been a no-op since 2b6eef94 (Make
apply --binary a no-op., 2006-09-06) and was deprecated in cb3a160d
(git-am: ignore --binary option, 2008-08-09).

We could remove it outright, but let's be nice to people who still
have scripts saying 'git am -b' (if they exist) and tell them the
reason for the sudden failure.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>