VENICE, Sept 5 (Reuters) - After the media was denied access to migrants desperately trying to cross the Polish-Belarus border, director Agnieszka Holland decided to step in and make a wrenching movie about their plight. "Green Border", which premieres at the Venice Film Festival on Tuesday, tells the story of refugees, charity workers, activists and border guards, whose lives intersect in the cold, swampy forests between the two east European countries. Migrants started flocking to the border in 2021, after Belarus, a close Russian ally, opened travel agencies in the Middle East offering a new unofficial route into Europe -...