bisect: drop unparse_commit() and use clear_commit_marks()

bisect: drop unparse_commit() and use clear_commit_marks()

The goal of this patch series is to check if good revisions are
ancestor of the bad revision without forking a process to launch
"git rev-list $good ^$bad".

This new version of this patch series does not use an "unparse_commit"
function anymore, we use "clear_commit_marks" instead.

Signed-off-by: Junio C Hamano <gitster@pobox.com>