More work on the sahpool digest fix. New/fixed versions can read legacy (no digest...

More work on the sahpool digest fix. New/fixed versions can read legacy (no digest) files but the reverse is only possible in limited circumstances (when files originated from a legacy version). The burning question is whether the real fix would be to remove this digest check altogether, as it only applies in a very limited context, and the fact that it was broken for some 18 months unnoticed suggests that its value might not be worth the CPU cycles.

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