TBILISI, Georgia — Seventy-five-year-old Marina Terishvili’s teenage son Mamuka was shot dead at a nationalist rally in Georgia in 1992. Now her other son, Giorgi, has been arrested for his role in protests against perceived Russian influence in their homeland. Seven police cars pulled up at her house in the capital Tbilisi on Friday and took Giorgi, a 52-year-old taxi driver, into custody, she said. He …