[GOLD] add new method for computing a build ID (take 2)

[Cary Coutant]

> During queue_final_tasks we don't know the size of string to be
> hashed, so perhaps the way to go is to create a ..._runner that is
> arranged to run just before Close_task_runner?
>
> Alternatively, we could decide to treehash or not before knowing the
> size of the string.

What do you mean by "size of the string to be hashed"? Looking at
start_asynchronous_build_id_if_needed, it wants to know the size of
the output file, which is determined by Layout::finalize before
Layout_task_runner::run calls queue_final_tasks. I think everything
you need to know is known at that point.

-cary