By Alan Charlish and Anna Wlodarczak-Semczuk WARSAW, Feb 9 (Reuters) - A Warsaw court ruled on Tuesday that two historians tarnished the memory of a Polish villager in a book about the Holocaust and must apologise, in a case some academics warn could deter impartial research into Poles' actions during World War Two. More than seven decades on, the conflict remains a live political issue in Poland, where the ruling nationalists say studies showing complicity by some Poles in the killing of Jews by Nazi Germany are an attempt to dishonour a country that suffered immensely in the conflict. The c...