"color.diff = true" is not "always" anymore.

"color.diff = true" is not "always" anymore.

Too many people got burned by setting color.diff and color.status to
true when they really should have set it to "auto".

This makes only "always" to do the unconditional colorization, and
change the meaning of "true" to the same as "auto": colorize only when
we are talking to a terminal.

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