bisect--helper: convert `vocab_*` char pointers to char arrays

bisect--helper: convert `vocab_*` 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 `vocab_bad` and `vocab_good` char pointers to char arrays.

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