The last few weeks have seen yet another flurry of mortgage mergers and spinoffs. Centex Corp. is selling its Centex Home Equity Co. to hedge fund Fortress Investment Group. Executives expect the sale of the subprime unit to close in 120 days. The purchase price will be book value plus a premium that the parties will calculate at closing. Centex estimates it will receive about $575 million in after-tax cash from the sale. …