bisect--helper: handle states directly

bisect--helper: handle states directly

In preparation for making `git bisect` a real built-in, let's prepare
the `bisect--helper` built-in to handle `git bisect--helper good` and
`git bisect--helper bad`, i.e. eliminate the need of `state` subcommand.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>