Fail with user friendly error when no new parent branch is specified.

tests/git_rebase_update_test.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.
 
 """Unit tests for git_rebase_update.py"""
 
 import os
 import sys
 
@@ skipping 134 matching lines @@
     self.assertIn('branch_L up-to-date', output)
     self.assertIn('foobar up-to-date', output)
     self.assertIn('sub_K up-to-date', output)
 
     with self.repo.open('bob') as f:
       self.assertEquals('testing auto-freeze/thaw', f.read())
 
     self.assertEqual(self.repo.git('status', '--porcelain').stdout, '?? bob\n')
 
     self.repo.git('checkout', 'origin/master')
+    _, err = self.repo.capture_stdio(self.rp.main, [])
+    self.assertIn('Must specify new parent somehow', err)
     _, err = self.repo.capture_stdio(self.rp.main, ['foobar'])
     self.assertIn('Must be on the branch', err)
 
     self.repo.git('checkout', 'branch_K')
     _, err = self.repo.capture_stdio(self.rp.main, ['origin/master'])
     self.assertIn('Cannot reparent a branch to its existing parent', err)
     output, _ = self.repo.capture_stdio(self.rp.main, ['foobar'])
     self.assertIn('Rebasing: branch_K', output)
     self.assertIn('Rebasing: sub_K', output)
     self.assertIn('Rebasing: branch_L', output)
@@ skipping 169 matching lines @@
     """)
 
 
 if __name__ == '__main__':
   sys.exit(coverage_utils.covered_main((
     os.path.join(DEPOT_TOOLS_ROOT, 'git_rebase_update.py'),
     os.path.join(DEPOT_TOOLS_ROOT, 'git_new_branch.py'),
     os.path.join(DEPOT_TOOLS_ROOT, 'git_reparent_branch.py'),
     os.path.join(DEPOT_TOOLS_ROOT, 'git_rename_branch.py')
   )))