Nothing someone could know about me? If someone knew everything there is to know about me, I sure hope they would be able to predict everything I will do. Free will doesn't mean freedom from my own nature.
The only way to be perfectly unpredictable is to act at random and without reason. If you act for a reason, then in principle, that reason could be known by someone else, and they could predict your actions. I think that's acceptable: I don't feel that in order for my choices to be free, my nature and my reasons must be unknowable to anyone but myself.
> But if that were possible and everything always went the same, then perhaps we could rule out free will
According to your definition, yes. Not according to mine. So I guess we made the OP's point ;)
The key difference is whether that reason is entirely internal to you or it's the result of external influence. If you always take a walk, an observer could infer that by some arcane law of physics, the sun forces you to take the walk - and they might not be far off.
What I mean by nondeterministic/random is there is nothing someone could know about you, the sun, and the rest of the universe that would let them predict with certainty whether you'll go for a walk or not.
The supernatural ability to rewind time seems like cheating, to be honest. But if that were possible and everything always went the same, then perhaps we could rule out free will (depending on the nature of this time reversal mechanism).