bisect: use "bisect--helper" and remove "filter_skipped" function

bisect: use "bisect--helper" and remove "filter_skipped" function

Use the new "git bisect--helper" builtin. It should be faster and
safer instead of the old "filter_skipped" shell function. And it
is a first step to move more shell code to C.

As the output is a little bit different we have to change the code
that interpret the results. But these changes improve code clarity.

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