bisect--helper: reimplement `bisect_skip` shell function in C

bisect--helper: reimplement `bisect_skip` shell function in C

Reimplement the `bisect_skip()` shell function in C and also add
`bisect-skip` subcommand to `git bisect--helper` to call it from
git-bisect.sh

Using `--bisect-skip` subcommand is a temporary measure to port shell
function to C so as to use the existing test suite.

Mentored-by: Lars Schneider <larsxschneider@gmail.com>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Pranit Bauva <pranit.bauva@gmail.com>
Signed-off-by: Tanushree Tumane <tanushreetumane@gmail.com>
Signed-off-by: Miriam Rubio <mirucam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>