One of the many signs that it’s Christmastime in the city is the sight, sound and scent of the city’s sidewalk tree vendors. The annual arrival of the (mostly) jovial tree purveyors reminds us that bell-ringing Santas, office secret Santas, and bar-crawling Santas aren’t far behind. Each year thousands of trees are sold to New Yorkers to help them deck the halls for the season. But what about the people who sell those trees? A new documentary film, “Tree Man,” gives us a peek at the lives of the city’s tree sellers, many of whom leave families behind to camp out in sometimes harsh living conditions for the sake of their business.