bisect: introduce --no-checkout support into porcelain.

bisect: introduce --no-checkout support into porcelain.

git-bisect can now perform bisection of a history without performing
a checkout at each stage of the bisection process. Instead, HEAD is updated.

One use-case for this function is allow git bisect to be used with
damaged repositories where git checkout would fail because the tree
referenced by the commit is damaged.

It can also be used in other cases where actual checkout of the tree
is not required to progress the bisection.

Improved-by: Christian Couder <chriscool@tuxfamily.org>
Improved-by: Junio C Hamano <gitster@pobox.com>
Improved-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>