Elon Musk just dropped a cool $1 billion on Tesla Inc TSLA stock, his first open-market buy since 2020 — the last time he did this, the stock went on a historic tear. But this time, the road ahead looks far less certain.Track TSLA stock here.Short-Term Pop, Familiar PlaybookInsider buying typically triggers immediate confidence and short covering, and Musk's purchase has already jolted Tesla's stock higher. The stock was already up 6% at the time of publication on Monday.Related: Tesla Stock Climbs Nearly 7 ...