bisect: check ancestors without forking a "git rev-list" process

bisect: check ancestors without forking a "git rev-list" process

We must save the pending commits that will be used during revision
walking and unparse them after, because we want to leave a clean
state for the next revision walking that will try to find the best
bisection point.

As we don't fork a process anymore to call "git rev-list", we need
to remove the use of GIT_TRACE to check how "git rev-list" is
called from the t6030 test that uses it.

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