For the first time since 1999, Ballet Theater is performing “Push Comes to Shove,” the first work Tharp made for the company, in 1976. Its original star was Baryshnikov, who had recently defected from the Soviet Union and was, for many, the most articulate ballet dancer ever. Tharp tested that technique by torquing it off balance, and she half-discovered, half-created a new character: an American Baryshnikov who demonstrated control by losing it and showed his cool by being a clown.