The Italian bank Unicredit is keeping its options open after taking a stake in the German Commerzbank, but has not ruled out a merger of the two institutions. “Commerzbank is an investment. Nothing else,” Unicredit chief executive Andrea Orcel said at a Bank of America conference in London on Wednesday. However, he said a merger with the Frankfurt-based company could become “a test case for Europe,” which he believes needs larger banks. “We can come together and do something greater.” Commerzbank is seen as a good strategic fit for the major Italian bank. The German banking market is fragmente…