The boutique investment management firm is being sold.Shares of Diamond Hill Investment Group (DHIL +44.40%) leaped 44% on Thursday after the asset manager struck a deal to be acquired by First Eagle Investments for $473 million. Diamond Hill's investment team is slated to stay on boardUnder the terms of the deal, First Eagle will purchase all of Diamond Hill's outstanding stock for$175 per share in cash. That's a premium of more than 49% compared to Diamond Hill's closing stock price on Dec. 10."This par ...