Martha Stewart can keep a secret. The home and garden guru revealed this week it was "very easy" to keep confidential the decades-old affair she recently revealed."You have to be circumspect," the 83-year-old told businessman Lee Brian Schrager while celebrating her 100th book Wednesday night. Schrager had asked how she kept the affair a secret all these years, joking it could be the subject of her next book.Video of the exchange was posted by the Daily Mail. MARTHA STEWART SAYS INA GARTEN STOPPED TALKING TO HER AFTER STEWART WENT TO PRISON Earlier this month, in the trailer for her new Netflix documentary, Stewart admitted