By Mitchell Forde We know who will start behind center for the San Francisco 49ers when they face the Dallas Cowboys in Sunday’s marquee NFL divisional playoff matchup. That will be Brock Purdy, the seventh-round rookie who has led the team to seven consecutive wins. The team ruled fellow quarterback Jimmy Garoppolo out for the matchup on Friday. But should the 49ers advance deeper into the postseason, it sounds possible that Purdy could have some competition. 49ers coach Kyle Shanahan said Saturday that Garoppolo, who has been sidelined since Week 13 due to a broken bone in his foot, could po…