bisect: libify `check_merge_bases` and its dependents

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.

Emulate try catch in C by converting `exit(<positive-value>)` to
`return <negative-value>`. Follow POSIX conventions to return
<negative-value> to indicate error.

In `check_merge_bases()` there is an early success special case,
so we have introduced special error code
BISECT_INTERNAL_SUCCESS_MERGE_BASE (-11) which indicates early
success. This BISECT_INTERNAL_SUCCESS_MERGE_BASE is converted back
to BISECT_OK (0) in `check_good_are_ancestors_of_bad()`.

Update all callers to handle the error returns.

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: Miriam Rubio <mirucam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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