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

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

Reimplement the `bisect_write` shell function in C and add a
`bisect-write` subcommand to `git bisect--helper` to call it from
git-bisect.sh

Using `--bisect-write` 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.

Note: bisect_write() uses two variables namely TERM_GOOD and TERM_BAD
from the global shell script thus we need to pass it to the subcommand
using the arguments. We then store them in a struct bisect_terms and
pass the memory address around functions.

Add a log_commit() helper function to write the contents of the commit message
header to a file which will be re-used in future parts of the code as
well.

Also introduce a function free_terms() to free the memory of `struct
bisect_terms` and set_terms() to set the values of members in `struct
bisect_terms`.

Helped-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
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>