Dreamgirls has the emotional wallop of an old-fashioned movie musical. But it’s also a good deal fresher than that “old-fashioned” tag might suggest.Based on a hit Broadway stage musical from 1981, the Golden Globes best-picture winner follows the career of a Motown trio (called the Dreamettes in the story, but clearly based on Diana Ross and her cohorts in the original Supremes). The film is great fun right from the start, with the Dreamettes (Beyoncé Knowles, Jennifer Hudson and Anika Noni Rose) getting their first big break singing back-up for sex-machine soul singer James “Thunder” Early (a knockout performance by Eddie Murphy). And it never lets up. The history of Motown music, along with the life story of Berry Gordy Jr., hovers in the margins throughout, and it all plays out as a lively variation on the classic backstage musical, a show-biz romance with black characters performing in a ’60s-and-’70s setting.