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Use correct occupancy in connected_threat()
When checking if a move defends the threatened piece we correctly remove from the occupancy bitboard the moved piece. This patch removes from the occupancy also the threatening piece so to consider the cases of moves that defend the threatened piece x-raying through the threat move. As example in this position: r3k2r/p1ppqp2/Bn4p1/3p1n2/4P1N1/5Q1P/PPP2P1P/R3K2R w KQkq - 1 10 The threat black move is dxe4. With this patch we include (and so don't prune) white's Bb7 that would be pruned otherwise. The number of affected position is very low, around 1% of cases, so we don't expect ELO changes, neverthless this is the logical and natural thing to do. Patch suggested by Hongzhicheng. new bench: 5323798
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