Sacha Baron Cohen's return to incognito trickery is, in current conditions, a little like pouring rubbing alcohol into the nation's open wounds.Employing the same ingenious commitment and subterfuge that made him famous in the guise of Ali G., Borat Sagdiyev and Bruno Gehard, Cohen now plays several characters in Showtime's "Who Is America?," which starts out seeming like another one of those naively altruistic shows that listens to ordinary people's widely varied political beliefs in hopes that we can better understand our differences.