Given that "git show" always shows some diff and does not walk the history
by default, it is natural to expect "git show --first-parent" to show the
difference between the given commit and its first parent.  It also would
be natural, given that "--cc" is the default, "git show -m" to show
pairwise difference from each of the parents.

We however always defaulted to --cc and there was no way to turn it off.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>