VIENNA (Reuters) - A 25-year-old Bosnian man has been arrested in the Austrian city of Graz on suspicion of planning an attack on its Christmas market, police said on Thursday.     The Muslim man was detained on Dec. 1 at an emergency shelter where others saw him watching videos about last year s attacks in Nice and Berlin on a computer, a force spokesman said. Witnesses heard him talking about Berlin and death, the spokesman added.  He had visited mosques, but it was not yet clear whether he had been radicalized or had any accomplices, police said. A Tunisian man killed 12 people by driving a truck into a Christmas market in Berlin a year ago. The failed asylum seeker had pledged allegiance to Islamic State. 