bisect--helper: introduce new `write_in_file()` function

bisect--helper: introduce new `write_in_file()` function

Let's refactor code adding a new `write_in_file()` function
that opens a file for writing a message and closes it and a
wrapper for writing mode.

This helper will be used in later steps and makes the code
simpler and easier to understand.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Miriam Rubio <mirucam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>