By Svea Herbst-Bayliss NEW YORK(Reuters) -Blackstone Group, the world's largest hedge fund investor, is committing $250 million in start-up capital to a hedge fund run by a former portfolio manager at Fir Tree Partners, two sources familiar with the matter said. Sachin Gupta launched Covara Capital, a new opportunistic long-short credit investment manager, with capital from its $1.1 billion Strategic Alliance Fund IV, which backs new entrants in the hedge fund industry, and from other pools of company ca ...