Turn `git bisect` into a full built-in

Turn `git bisect` into a full built-in

Now that the shell script hands off to the `bisect--helper` to do
_anything_ (except to show the help), it is but a tiny step to let the
helper implement the actual `git bisect` command instead.

This retires `git-bisect.sh`, concluding a multi-year journey that many
hands helped with, in particular Pranit Bauna, Tanushree Tumane and
Miriam Rubio.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>