bisect: make "git bisect" use new "--next-all" bisect-helper function

bisect: make "git bisect" use new "--next-all" bisect-helper function

This patch replace the "--next-exit" option of "git bisect--helper"
with a "--next-all" option that does merge base checking using
the "check_good_are_ancestors_of_bad" function implemented in
"bisect.c" in a former patch.

The new "--next-all" option is then used in "git-bisect.sh" instead
of the "--next-exit" option, and all the shell functions in
"git-bisect.sh" that are now unused are removed.

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