bisect--helper: reimplement `bisect_next` and `bisect_auto_next` shell functions...

bisect--helper: reimplement `bisect_next` and `bisect_auto_next` shell functions in C

Reimplement the `bisect_next()` and the `bisect_auto_next()` shell functions
in C and add the subcommands to `git bisect--helper` to call them from
git-bisect.sh .

bisect_auto_next() function returns an enum bisect_error type as whole
`git bisect` can exit with an error code when bisect_next() does.

Return an error when `bisect_next()` fails, that fix a bug on shell script
version.

Using `--bisect-next` and `--bisect-auto-next` subcommands is a
temporary measure to port shell function to C so as to use the existing
test suite. As more functions are ported, `--bisect-auto-next`
subcommand will be retired and will be called by some other methods.

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>