am: officially deprecate -b/--binary option

We have had these options as harmless no-op for more than 3 years without
officially deprecating them.  Let's announce the deprecation and start
warning against their use, but without failing the command just not yet,
so that we can later repurpose the option if we want to in the future.

Signed-off-by: Junio C Hamano <gitster@pobox.com>