Is the stock a buy?Walmart (WMT 0.83%) shares rallied following the company's first-quarter results, and the stock is now up more than 20% year to date. However, more upside could be in store for the stock.Let's look at the retail giant's most recent quarterly results and the one thing it can do to continue this rally and take the stock to new heights.Impressive resultsWalmart turned in impressive Q1 results. Its revenue rose 6% to $161.5 billion and its adjusted earnings per share (EPS) climbed 22$0.6 ...