bisect--helper: `get_terms` & `bisect_terms` shell function in C

bisect--helper: `get_terms` & `bisect_terms` shell function in C

Reimplement the `get_terms` and `bisect_terms` shell function in C and
add `bisect-terms` subcommand to `git bisect--helper` to call it from
git-bisect.sh .

Using `--bisect-terms` subcommand is a temporary measure to port shell
function in C so as to use the existing test suite. As more functions
are ported, this subcommand will be retired but its implementation will
be called by some other methods.

Also use error() to report "no terms defined" and accordingly change the
test in t6030.

We need to use PARSE_OPT_KEEP_UNKNOWN here to allow for parameters that
look like options (e.g --term-good) but should not be parsed by
cmd_bisect__helper(). This change is safe because all other cmdmodes have
strict argc checks already.

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: Junio C Hamano <gitster@pobox.com>