bisect: libify `bisect_checkout`

bisect: libify `bisect_checkout`

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.

Turn `exit()` to `return` calls in `bisect_checkout()`.
Changes related to return values have no bad side effects on the
code that calls `bisect_checkout()`.

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