discovery: Fix error print mentioning a 'None' branch

When talking to old server discovery use heads of the 'None' branch to refer to
topological heads. This use of None should never make it to the end user.

mercurial/discovery.py

@@ -161,7 +161,7 b' def prepush(repo, remote, force, revs, n'
             if len(newhs) > len(oldhs):
                 dhs = list(newhs - oldhs)
                 if error is None:
-                    if branch != 'default':
+                    if branch not in ('default', None):
                         error = _("push creates new remote head %s "
                                   "on branch '%s'!") % (short(dhs[0]), branch)
                     else:
@@ -173,7 +173,8 b' def prepush(repo, remote, force, revs, n'
                     else:
                         hint = _("did you forget to merge? "
                                  "use push -f to force")
-                repo.ui.note("new remote heads on branch '%s'\n" % branch)
+                if branch is not None:
+                    repo.ui.note("new remote heads on branch '%s'\n" % branch)
                 for h in dhs:
                     repo.ui.note("new remote head %s\n" % short(h))
         if error: