bug in rewindHeaderState triggered by EBBs

[nfrisby]

https://github.com/input-output-hk/ouroboros-network/blob/599afc730af5f8d64accc6ab1c8d05bdb27ee7d8/ouroboros-consensus/src/Ouroboros/Consensus/HeaderValidation.hs#L200-L209

If p is an EBB with a proper block in the same slot, then the annTipPoint t > p is correct only half of the time: it will either correctly rewind the tips to the EBB or it will incorrectly also retain the proper block, depending on how the hashes happen to compare.

[edsko]

#@!%!@#!@%!@ EBBs @#!#@^#!@#!@%!@#u@$*&@#*@&!#!

I'm thinking maybe there is an easier solution here. Rewinding is only allowed to rewind to a point previously applied right? So perhaps we can just say

    rolledBack :: AnnTip blk -> Bool
    rolledBack t = annTipPoint  t /= p

instead? We are dropping from the right after all. @nfrisby , @mrBliss , what do you think?

[mrBliss]

I like Edsko's proposed fix, it's simple.