git-rebase--interactive.sh: Make 3-way merge strategies work for -p.

git-rebase--interactive.sh: Make 3-way merge strategies work for -p.

git-rebase--interactive.sh used to pass all parents of a merge commit to
git-merge, which means that we have at least 3 heads to merge: HEAD,
first parent and second parent. So 3-way merge strategies like recursive
wouldn't work.

Fortunately, we have checked out the first parent right before the merge
anyway, so that is HEAD. Therefore we can drop simply it from the list
of parents, making 3-way strategies work for merge commits with only
two parents.

Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>