reachableroots: sort the smartset in the pure version too

Changeset be8a4e0800d8 uses smartset lazy sorting for the C version. We need to
apply the same to the pure version for consistency. This is fixing the tests
with --pure.

mercurial/revset.py

@@ -123,7 +123,9 def reachablerootspure(repo, minroot, ro
         for parent in seen[rev]:
             if parent in reachable:
                 reached(rev)
-    return baseset(sorted(reachable))
+    reachable = baseset(reachable)
+    reachable.sort()
+    return reachable
 
 def reachableroots(repo, roots, heads, includepath=False):
     """return (heads(::<roots> and ::<heads>))