In a first for a mayor of Boston, Michelle Wu visited Nova Scotia last week to help cut down the 45-foot-tall white spruce evergreen Christmas tree that arrived by police escort at Boston Common on Tuesday. The people of the province send a holiday tree to Boston annually as a thank-you gift for Boston’s emergency assistance following a devastating munitions explosion in Halifax Harbor in 1917. The city’s official tree-lighting event will be held on the Common on Thurs., Dec. 4 from 6 p.m. to 8 p.m. It will be broadcast live on WCVB Channel 5 starting at 7 p.m. City of Boston photo…