Warren Buffett’s surprise investment in Home Capital Group Inc. all started with a single email from a Canadian investment banker who first met the billionaire 23 years ago. Don Johnson, 82, an advisory board member at Bank of Montreal’s BMO Capital Markets unit, reached out to Buffett in a June 9 email, adding a photo of the two of them to remind Berkshire Hathaway Inc.’s chairman of their past connections. “I just brought to his attention this Home Capital opportunity," Johnson said of his message to Buffett. “I informed him that BMO Capital Markets had been retained as one of...