DirState.update_basis_by_delta checks Tree[0]

[Robert Collins]

On Thu, May 19, 2011 at 6:07 PM, John Arbash Meinel
<john at arbash-meinel.com> wrote:
> It is a bit tricky to get right, but since I've seen the perf results, I
> feel obligated to finish this off. :)

Cool; I nearly followed up my mail saying 'and will be
wrong/incomplete for the other cases' but I figured that would be
evident from the focus on commit.

-Rob