Add better error messages to git_reparent_branch.

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 369 matching lines @@
     """)
 
     output, _ = self.repo.capture_stdio(self.rp.main, ['--root'])
     self.assertIn('to track origin/master (was lkgr [tag])', output)
 
     self.assertSchema("""
     A B C D E F G M N O foobar1 foobar2
                   M H I J K L
     """)
 
+  def testReparentBranchWithoutUpstream(self):
+    self.repo.git('branch', 'nerp')
+    self.repo.git('checkout', 'nerp')
+
+    _, err = self.repo.capture_stdio(self.rp.main, ['branch_K'])
+
+    self.assertIn('Unable to determine nerp@{upstream}', err)
+
 
 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')
   )))