Ashley Olsen is back on the market. She and her boyfriend, Tsubi jeans czar Greg Chait, have split after an 18-month relationship. The reason: He wouldn’t commit. “When they were together over the holidays, she told him she was looking for something more permanent. He wasn’t ready for that,” a source close to Chait told us. “So they decided on an amicable parting.” A rep for Olsen had no comment.