diff-merges: introduce '--dd' option

diff-merges: introduce '--dd' option

This option provides a shortcut to request diff with respect to first
parent for any kind of commit, universally. It's implemented as pure
synonym for "--diff-merges=first-parent --patch".

Gives user quick and universal way to see what changes, exactly, were
brought to a branch by merges as well as by regular commits.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>