git-bisect: allow bisecting with only one bad commit.

git-bisect: allow bisecting with only one bad commit.

This allows you to say:

	git bisect start
	git bisect bad $bad
	git bisect next

to start bisection without knowing a good commit.  This would
have you try a commit that is half-way since the beginning of
the history, which is rather wasteful if you already know a good
commit, but if you don't (or your history is short enough that
you do not care), there is no reason not to allow this.

Signed-off-by: Junio C Hamano <junkio@cox.net>