revset: gratuitous code move in '_children'

As 'cs' is empty as the time of the conditional, we can just return an empty
'baseset' and create the variable later.

mercurial/revset.py

@@ -614,9 +614,9 b' def checkstatus(repo, subset, pat, field'
     return subset.filter(matches)
 
 def _children(repo, narrow, parentset):
+    if not parentset:
+        return baseset()
     cs = set()
-    if not parentset:
-        return baseset(cs)
     pr = repo.changelog.parentrevs
     # XXX this should be 'parentset.min()' assuming 'parentset' is a smartset
     # (and if it is not, it should.)