bisect--helper: `check_and_set_terms` shell function in C

bisect--helper: `check_and_set_terms` shell function in C

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

Using `--check-and-set-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.

check_and_set_terms() sets and receives two global variables namely
TERM_GOOD and TERM_BAD in the shell script. Luckily the file BISECT_TERMS
also contains the value of those variables so its appropriate to evoke the
method get_terms() after calling the subcommand so that it retrieves the
value of TERM_GOOD and TERM_BAD from the file BISECT_TERMS. The two
global variables are passed as arguments to the subcommand.

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>