Tesla shares surged on Monday, extending gains since Elon Musk called a surprise all-hands meeting with employees last Thursday.CEO Elon Musk encouraged employees to hold on to their shares, which had fallen more than 50% from their all-time high in December.Retail investors have been buying Tesla stock at high levels during the recent sell-off.JPMorgan reported that retail traders have poured $7.3 billion into Tesla stock in the past two weeks.