diff-merges: let new options enable diff without -p

diff-merges: let new options enable diff without -p

New options don't have any visible effect unless -p is either given or
implied, as unlike -c/-cc we don't imply -p with --diff-merges. To fix
this, this patch adds new functionality by letting new options enable
output of diffs for merge commits only.

Add 'merges_need_diff' field and set it whenever diff output for merges is
enabled by any of the new options.

Extend diff output logic accordingly, to output diffs for merges when
'merges_need_diff' is set even when no -p has been provided.

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