fix set copy bug in GameState.copy()
fix set copy bug in GameState.copy()

self.group_sets and self.liberty_sets should share the underlying sets
between stones in the same group.  This patch makes this the case for
BoardState copies.

Rochester-NRT/AlphaGo,Rochester-NRT/AlphaGo,MaMiFreak/RocAlphaGo,hiropppe/RocAlphaGo,wrongu/RocAlphaGo,Rochester-NRT/AlphaGo,vftens/RocAlphaGo,Rochester-NRT/RocAlphaGo