changegroupsubset: remove unneeded cast to list

mercurial/localrepo.py

@@ -1313,7 +1313,6 b' class localrepository(repo.repository):'
         for n in bases:
             knownheads.update(cl.parents(n))
         knownheads.discard(nullid)
-        knownheads = list(knownheads)
         if knownheads:
             # Now that we know what heads are known, we can compute which
             # changesets are known.  The recipient must know about all