bisect--helper: rewrite `check_term_format` shell function in C

bisect--helper: rewrite `check_term_format` shell function in C

Reimplement the `check_term_format` shell function in C and add
a `--check-term-format` subcommand to `git bisect--helper` to call it
from git-bisect.sh

Using `--check-term-format` subcommand is a temporary measure to port
shell function to C so as to use the existing test suite. As more
functions are ported, this subcommand will be retired and its
implementation will be called by some other method/subcommand. For
eg. In conversion of write_terms() of git-bisect.sh, the subcommand will
be removed and instead check_term_format() will be called in its C
implementation while a new subcommand will be introduced for write_terms().

Helped-by: Johannes Schindelein <Johannes.Schindelein@gmx.de>
Mentored-by: Lars Schneider <larsxschneider@gmail.com>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Pranit Bauva <pranit.bauva@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>