combine-diff: extend --cc logic to Octopus.

combine-diff: extend --cc logic to Octopus.

Santi Bejar points out that a hunk that changes from all the
same common parents except one is uninteresting.  The earlier
round marked changes from only one parent uninteresting, but
this also marks hunks that have the same change from all but one
parent uninteresting, which is a natural extension of the
original idea to Octopus merges.

Signed-off-by: Junio C Hamano <junkio@cox.net>