tahoe_cp doesn't correctly determine when it needs to copy bytes

[itamarst]

tahoe_cp appears to have optimizations to skip copying bytes in some cases.

However, it's buggy: it does if source.need_to_copy_bytes: (see implementation of Copier.need_to_copy_bytes) but need_to_copy_bytes is a method! So it always comes out True, instead of calling the method and seeing the response.

If that bug is fixed it hits some currently untested code paths and uncovers some bugs. So fixing this would also involve fixing those currently unreachable optimized code paths.