It's a battle between a big stock market winner and a potential turnaround opportunity.It's a matchup between the world's largest retailer and its smaller discount store rival. In one corner, Walmart (WMT -0.64%) stock is having a banner year, up 54% amid a string of better-than-expected results. On the other side, Dollar General (DG -1.25%) has struggled to manage weak sales, sending shares lower by 51% year to date.What does this night-and-day difference in the performance between these two consumer goods ...