BARTELLA, Iraq — For the 300 Christians who braved rain and wind to attend Christmas’s Eve Mass in their hometown, the ceremony evoked both holiday cheer and grim reminders of the war raging around their northern Iraqi town, and the distant prospect of moving back home. Displaced when the Islamic State of Iraq and Syria seized their town, Bartella, in August 2014, the Christians were bused into town from Irbil, capital of the self-ruled Kurdish region where they have lived for more than two years, to attend the lunchtime service in the Assyrian Orthodox church of Mart Shmoni.