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

git_nav_downstream.py

 #!/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
 which branch.
 """
 import sys
 
 from git_common import current_branch, branches, upstream, run, hash_one
 
 
 def main(argv):
   assert len(argv) == 1, "No arguments expected"
   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:
     return "No downstream branches"
   elif len(downstreams) == 1:
     run('checkout', downstreams[0])
   else:
     high = len(downstreams) - 1
     print
     while True:
       print "Please select a downstream branch"
       for i, b in enumerate(downstreams):
         print "  %d. %s" % (i, b)
       r = raw_input("Selection (0-%d)[0]: " % high).strip() or '0'
       if not r.isdigit() or (0 > int(r) > high):
         print "Invalid choice."
       else:
         run('checkout', downstreams[int(r)])
         break
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))