discovery: Fix error print mentioning a 'None' branch stable

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.

--- a/mercurial/discovery.py	Mon Oct 17 14:14:28 2011 -0500
+++ b/mercurial/discovery.py	Mon Oct 17 11:37:59 2011 +0200
@@ -161,7 +161,7 @@
             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 @@
                     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: