[m-rev.] for post-commit review: compute NeedsUpdate directly

[Zoltan Somogyi]

For post-commit review by Peter. I would like to hear whether
you know the reason for the change of semantics mentioned
in an XXX comment, and whether the other issues mentioned
in that comment are known limitations or not.

An earlier version of this diff checked whether the new, direct
algorithm computed the same answers as the old, indirect one;
it did throughout a full bootcheck, including the pack_args_reuse
test case.

Zoltan.