Citigroup to Buy Texas Bank   NEW YORK (Reuters) - Citigroup Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=C.N target=/stocks/quickinfo/fullquote"&gt;C.N&lt;/A&gt; the world's  largest financial services company, said on Tuesday it will  acquire Texas-based First American Bank.