Elon Musk responded to an unprecedented pay proposal from Tesla Inc.’s board by buying about $1 billion worth of shares, sending the carmaker’s stock soaring into positive territory for the year. The billionaire bought the shares indirectly through a revocable trust on Sept. 12, according to a regulatory filing released Monday. The purchase coincided with Tesla Chair Robyn Denholm speaking with reporters about the merits of awarding Musk around$1 trillion worth of stock if the company achieves a series ...