exchange: allow fallbackheads to use lazy set behavior The common ancestor set implementation was made lazy a couple years ago, but this piece of code still required processing the entire repo by putting set() around the lazy set. The code was introduced in 5653f2d166ea, a year before the lazy ancestor set was added. Dropping the set() shaves 3.5 seconds off of 'push -r' in repos with hundreds of thousands of commits.