wireprotocol: use visibleheads as reference while unbundling (issue 3303)

The `repo` object here is *always* local. Using `repo.heads()` ensure we will
reject push if any secret changeset exists.

During discovery, `visibleheads` were sent to the peer. So we can only expect it
to send us `visibleheads` back. If any secret changeset exists::

    visibleheads != repo.heads()

This fix server side part of issue 3303 when pushing over the wire.

mercurial/wireproto.py

@@ -555,7 +555,7 b' def unbundle(repo, proto, heads):'
     their_heads = decodelist(heads)
 
     def check_heads():
-        heads = repo.heads()
+        heads = phases.visibleheads(repo)
         heads_hash = util.sha1(''.join(sorted(heads))).digest()
         return (their_heads == ['force'] or their_heads == heads or
                 their_heads == ['hashed', heads_hash])