bisect--helper: convert `*_warning` char pointers to char arrays.

bisect--helper: convert `*_warning` char pointers to char arrays.

Instead of using a pointer that points at a constant string,
just give name directly to the constant string; this way, we
do not have to allocate a pointer variable in addition to
the string we want to use.

Let's convert `need_bad_and_good_revision_warning` and
`need_bisect_start_warning` char pointers to char arrays.

Mentored-by: Christian Couder <redacted>
Signed-off-by: Tanushree Tumane <redacted>
Signed-off-by: Miriam Rubio <redacted>
Signed-off-by: Junio C Hamano <redacted>