The hedge-fund manager, David Abrams, has personally become a billionaire, and earned billions more for his wealthy investors, over the past five years running what is effectively a one-man shop, according to company and investor documents reviewed by The Wall Street Journal and people who have worked with him. His firm, Abrams Capital Management LP, manages nearly $8 billion across three funds and is discussing raising money for a fourth fund that could help push its assets past $10 billion.