(petitioner entered into installment land contract in 2007 and gets deed to property, including home, in 2008 when petitioner took out conventional mortgage and paid off seller; petitioner claimed first-time homebuyer tax credit; IRS denies credit because purchase occurred in 2007 pre-dating existence of credit; petitioner had only had equitable title in 2007 under state (IA) law and was in possession of home and bore benefits and burdens of ownership; petitioner deducted mortgage interest and real estate taxes in 2007, again before advent of credit; IRS position upheld).