diff-merges: do not imply -p for new options

diff-merges: do not imply -p for new options

Add 'combined_imply_patch' field and set it only for old --cc/-c
options, then imply -p if this flag is set instead of implying -p
whenever 'combined_merge' flag is set.

We don't want new --diff-merge options to imply -p, to make it
possible to enable output of diffs for merges independently from
non-merge commits. At the same time we want to preserve behavior of
old --c/-c/-m options and their interactions with --first-parent, to
stay backward-compatible.

This patch is first step in this direction: it separates old "--cc/-c
imply -p" logic from the rest of the options.

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