The sigh could be heard at the Ohio state line Monday night when the Giants’ Double-A affiliate in Richmond, Va., ended a 15-game losing streak with a 5-1 victory at Altoona, Pa. Starter Kelvin Marte guided the Flying Squirrels to their fourth win in 21 games by allowing one run in seven innings. Even in the minors, where development supersedes records, a losing streak that long is troubling. “It’s tough on a club,” Giants general manager Bobby Evans said.