diff-merges: implement new values for --diff-merges

diff-merges: implement new values for --diff-merges

We first implement new options as exact synonyms for their original
counterparts, to get all the infrastructure right, and keep functional
improvements for later commits.

The following values are implemented:

--diff-merges=	        old equivalent
first|first-parent    = --first-parent (only format implications)
sep|separate          = -m
comb|combined         = -c
dense| dense-combined = --cc

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