builtin rebase: support --force-rebase

builtin rebase: support --force-rebase

In this commit, we add support to `--force-rebase` option. The
equivalent part of the shell script found in `git-legacy-rebase.sh` is
converted as faithfully as possible to C.

The --force-rebase option ensures that the rebase does not simply
fast-forward even if it could.

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>