Fix git_nav_downstream.py when origin/master is checked out.

git_nav_downstream.py

Index: git_nav_downstream.py
diff --git a/git_nav_downstream.py b/git_nav_downstream.py
index 9c2a53780a9f288376c16c3ecefffa6f0ca948c8..e394343ba4ba3bd75f3648c2e50f171c5530d74d 100755
--- a/git_nav_downstream.py
+++ b/git_nav_downstream.py
@@ -1,4 +1,8 @@
 #!/usr/bin/env python
+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
 """
 Checks out a downstream branch from the currently checked out branch. If there
 is more than one downstream branch, then this script will prompt you to select
@@ -14,7 +18,11 @@ def main(argv):
   upfn = upstream
   cur = current_branch()
   if cur == 'HEAD':
-    upfn = lambda b: hash_one(upstream(b))
+    def _upfn(b):
+      parent = upstream(b)
+      if parent:
+        return hash_one(parent)
+    upfn = _upfn
     cur = hash_one(cur)
   downstreams = [b for b in branches() if upfn(b) == cur]
   if not downstreams: