git-rev-list: add "--bisect" flag to find the "halfway" point

git-rev-list: add "--bisect" flag to find the "halfway" point

This is useful for doing binary searching for problems.  You start with
a known good and known bad point, and you then test the "halfway" point
in between:

	git-rev-list --bisect bad ^good

and you test that.  If that one tests good, you now still have a known
bad case, but two known good points, and you can bisect again:

	git-rev-list --bisect bad ^good1 ^good2

and test that point.  If that point is bad, you now use that as your
known-bad starting point:

	git-rev-list --bisect newbad ^good1 ^good2

and basically at every iteration you shrink your list of commits by
half: you're binary searching for the point where the troubles started,
even though there isn't a nice linear ordering.