ON THE OPENING NIGHT OF every Broadway musical, a constant flow of flowers, balloons and gifts swirls past the stage door and into the theater. As the presents begin to spill into the corridors outside the dressing rooms, the oldest and most beloved gift arrives: the Gypsy Robe.The robe is Broadway history made visible. It travels from musical to musical in a tradition stretching back more than 45 years. Every Broadway show with a chorus of dancers and singers, known in the business as gypsies, takes part when its opening night arrives.