Freddie Mac this year has been slowly winning back market share it lost to rival Fannie Mae, thanks to an active pursuit of portfolio growth and expanding securitization appetite. Fannie still holds a clear edge in total MBS issuance, with $134.07 billion through the first four months of 2005. But the company’s share of the GSE mortgage securitization market has declined from 59.0 percent during the first four months of 2004 to 55.7 percent during…