Fidelity marked down its stake in Twitter by another 7.9%, the mutual fund giant reported in a monthly update, bringing its total writedown on its stake since its acquisition to 63.5%. Fidelity was one of a number of investors which backed Elon Musk’s $44 billion buyout last October. Its writedowns reflect how the value of the equity that Musk and his backers got has shrunk since the deal,...