Red Hat Inc. will buy FuseSource Corp., an open source software company that is a subsidiary of Bedford’s Progress Software Corp., for an undisclosed sum. Progress Software is one of the Bay State’s oldest technology companies. It has cut its workforce and product lines. Red Hat has its technology and products division in Westford. It provides open-source Linux operating software. A Progress spokeswoman said the sale is part of its effort to focus on its core business.