Bisect: teach "bisect start" to optionally use one bad and many good revs.

Bisect: teach "bisect start" to optionally use one bad and many good revs.

One bad commit is fundamentally needed for bisect to run,
and if we beforehand know more good commits, we can narrow
the bisect space down without doing the whole tree checkout
every time we give good commits.

This patch implements:

    git bisect start [<bad> [<good>...]] [--] [<pathspec>...]

as a short-hand for this command sequence:

    git bisect start
    git bisect bad $bad
    git bisect good $good1 $good2...

On the other hand, there may be some confusion between revs
(<bad> and <good>...) and <pathspec>... if -- is not used
and if an invalid rev or a pathspec that looks like a rev is
given.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>