As a group, hedge funds have been backing away from the stock market even as its dramatic rally added trillions of dollars to share values since Christmas. But one aspect of their faith is rising: the belief that the stocks they do own are the right ones. It’s visible in a measure of concentration that plots how much of the average hedge fund portfolio is dominated by its biggest holdings. According to quarterly filings compiled by Goldman Sachs, the top 10 holdings on average made up 70 percent of a fund’s long portfolios, the highest since at least 2002.