Citigroup Inc. C Chief Financial Officer Mark Mason said Wednesday that the bank’s markets revenue was down 20% so far in the second quarter, but added that the company wasn’t changing its full-year outlook. Mason said that Citigroup continued “to have very good corporate engagement, corporate dialogue,” but added that “the debt ceiling concerns certainly did weigh on the investor client base in particular in April and May.” However, he said that the early stretch of this month had been quieter, in terms of market volatility. Despite shakier trends in the company’s markets and investment-banking businesses, Mason said…