This commit adds support for `--fork-point` and `--no-fork-point`.
This is converted as-is from `git-legacy-rebase.sh`.
Signed-off-by: Pratik Karki <predatoramigo@gmail.com> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>