NEW YORK — Citigroup on Friday demanded that Wachovia call off its proposed $15.1 billion merger with Wells Fargo, saying it has an exclusive deal to buy the Charlotte-based bank. Wachovia's announcement early Friday of the all-stock deal with San Francisco-based Wells Fargo & Co. was an abrupt reversal that trumped Citigroup's plan to acquire Wachovia's banking operations with government assistance. The Citigroup deal would have been done with the help of the Federal Deposit Insurance Corp., but the Wells Fargo deal for Wachovia will be done without it. Shares of Wachovia and Wells rose in morning trading, while Citigroup...