diff-options: unconfuse description of --color

diff-options: unconfuse description of --color

It said "by default it is off" while it also said "the default is
always", which confused everybody who read it only once.  It wanted
to say (1) if you do not say --color, it is not enabled, and (2) if
you say --color but do not say when to enable it, it will always be
enabled".

Rephrase to clarify by using "default" only once.

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