revset: refactor parents() into a single return point

Both paths are doing similar thing in the end. We refactor the function so that
the `ps` set is commonly used at the end.

This will end excluding `nullrev` from this set in a future patch

mercurial/revset.py

@@ -1231,9 +1231,8 b' def parents(repo, subset, x):'
     The set of all parents for all changesets in set, or the working directory.
     """
     if x is None:
-        ps = tuple(p.rev() for p in repo[x].parents())
-        return subset & ps
-
+        ps = set(p.rev() for p in repo[x].parents())
+    else:
     ps = set()
     cl = repo.changelog
     for r in getset(repo, spanset(repo), x):