bisect--helper: add "--next-exit" to output bisect results

bisect--helper: add "--next-exit" to output bisect results

The goal of this patch is to port more shell code from the "bisect_next"
function in "git-bisect.sh" to C code in "builtin-bisect--helper.c".

So we port the code that interprets the bisection result and stops or
continues (by checking out the next revision) the bisection process.

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