Before-the-Bell: Wal-Mart Edges Lower  NEW YORK (Reuters) - Shares of Wal-Mart Stores Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=WMT.N target=/stocks/quickinfo/fullquote"&gt;WMT.N&lt;/A&gt;  ticked lower on Monday after the world's largest retailer cut  its August sales forecast.