bisect: limit the searchspace by pathspecs

bisect: limit the searchspace by pathspecs

It was surprisingly easy to do.

	git bisect start <pathspec>

followed by all the normal "git bisect good/bad" stuff.

Almost totally untested, and I guarantee that if your pathnames have
spaces in them (or your GIT_DIR has spaces in it) this won't work. I don't
know how to fix that, my shell programming isn't good enough.

This involves small changes to make "git-rev-list --bisect" work in the
presense of a pathspec limiter, and then truly trivial (and that's the
broken part) changes to make "git bisect" save away and use the pathspec.

I tried one bisection, and a "git bisect visualize", and it all looked
correct. But hey, don't be surprised if it has problems.

		Linus

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