rebase --rebase-merges: root commits can be cousins, too

Reported by Wink Saville: when rebasing with no-rebase-cousins, we
will want to refrain from rebasing all of them, even when they are
root commits.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>