rebase --rebase-merges: add support for octopus merges

rebase --rebase-merges: add support for octopus merges

Previously, we introduced the `merge` command for use in todo lists,
to allow to recreate and modify branch topology.

For ease of implementation, and to make review easier, the initial
implementation only supported merge commits with exactly two parents.

This patch adds support for octopus merges, making use of the
just-introduced `-F <file>` option for the `git merge` command: to keep
things simple, we spawn a new Git command instead of trying to call a
library function, also opening an easier door to enhance `rebase
--rebase-merges` to optionally use a merge strategy different from
`recursive` for regular merges: this feature would use the same code
path as octopus merges and simply spawn a `git merge`.

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