Activist investor Bill Ackman, the billionaire CEO of hedge fund Pershing Square Capital, is making a huge comeback — and he credits it to an insight he gleaned from years of studying his “mentor” Warren Buffett. After several years of underperformance, Pershing Square is up more than 40% year-to-date. Yet Ackman isn't actively raising capital, even after four years of the fund’s consecutive negative returns and significant investor redemptions. He may have the Oracle of Omaha to thank.