bisect: add enum to represent bisect returning codes

bisect: add enum to represent bisect returning codes

Since we want to get rid of git-bisect.sh, it would be necessary to
convert those exit() calls to return statements so that errors can be
reported.

Create an enum called `bisect_error` with the bisecting return codes
to use in `bisect.c` libification process.

Change bisect_next_all() to make it return this enum.

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