Jerry Falwell and an army of conservative lawyers drew a line in the snow yesterday, warning Boston and other cities: Don’t mess with Christmas. And even before the first volley was fired, City Hall caved. Just after a top lawyer for the conservative Liberty Counsel blasted the Hub’s annual “holiday tree” ceremony, a Menino administration official scrambled to do damage control. “This is a Christmas tree,” Boston Parks Commissioner Toni Pollak insisted about the Nova Scotia spruce the city will light Dec. 1 on historic Boston Common. “It’s definitely a Christmas tree.” However, the city’s official Web site refers to...