General Motors caught a break this week in its increasingly desperate hunt for cash when it announce a deal with an investor group to sell a 78 percent stake in GMAC Commercial Holding Corp., the automakers’ commercial mortgage unit. Kohlberg Kravis Roberts & Co., along with partners Five Mile Capital Partners and Goldman Sachs Capital Partners, agreed to buy the stake in a deal worth $8.8 billion. The unit already has a new name, Capmark…