Add better error messages to git_reparent_branch.

git_reparent_branch.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.
 
 """Change the upstream of the current branch."""
 
 import argparse
 import sys
 
@@ skipping 13 matching lines @@
                  help='New parent branch (or tag) to reparent to.')
   g.add_argument('--root', action='store_true',
                  help='Reparent to the configured root branch (%s).' % root_ref)
   g.add_argument('--lkgr', action='store_true',
                  help='Reparent to the lkgr tag.')
   opts = parser.parse_args(args)
 
   # TODO(iannucci): Allow specification of the branch-to-reparent
 
   branch = current_branch()
+
   if opts.root:
     new_parent = root_ref
   elif opts.lkgr:
     new_parent = 'lkgr'
   else:
     if not opts.new_parent:
       parser.error('Must specify new parent somehow')
     new_parent = opts.new_parent
   cur_parent = upstream(branch)
 
   if branch == 'HEAD' or not branch:
     parser.error('Must be on the branch you want to reparent')
   if new_parent == cur_parent:
     parser.error('Cannot reparent a branch to its existing parent')
 
+  if not cur_parent:
+    msg = (
+      "Unable to determine %s@{upstream}.\n\nThis can happen if you didn't use "
+      "`git new-branch` to create the branch and haven't used "
+      "`git branch --set-upstream-to` to assign it one.\n\nPlease assign an "
+      "upstream branch and then run this command again."
+    )
+    print >> sys.stderr, msg % branch
+    return 1
+
   mbase = get_or_create_merge_base(branch, cur_parent)
 
   all_tags = tags()
   if cur_parent in all_tags:
     cur_parent += ' [tag]'
 
   try:
     run('show-ref', new_parent)
   except subprocess2.CalledProcessError:
     print >> sys.stderr, 'fatal: invalid reference: %s' % new_parent
@@ skipping 14 matching lines @@
   # TODO(iannucci): ONLY rebase-update the branch which moved (and dependants)
   return git_rebase_update.main(['--no-fetch'])
 
 
 if __name__ == '__main__':  # pragma: no cover
   try:
     sys.exit(main(sys.argv[1:]))
   except KeyboardInterrupt:
     sys.stderr.write('interrupted\n')
     sys.exit(1)