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.

mercurial/exchange.py

@@ -147,7 +147,7 b' class pushoperation(object):'
         #
         # We can pick:
         # * missingheads part of common (::commonheads)
-        common = set(self.outgoing.common)
+        common = self.outgoing.common
         nm = self.repo.changelog.nodemap
         cheads = [node for node in self.revs if nm[node] in common]
         # and