old_contents: #!/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 argparse
  import sys
  
  from git_common import current_branch, branches, upstream, run_stream, hash_one
  
  
  def main(args):
    parser = argparse.ArgumentParser()
    parser.add_argument('--pick',
                        help=(
                            'The number to pick if this command would '
                            'prompt'))
    opts = parser.parse_args(args)
  
    upfn = upstream
    cur = current_branch()
    if cur == 'HEAD':
      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_stream('checkout', downstreams[0], stdout=sys.stdout, stderr=sys.stderr)
    else:
      high = len(downstreams) - 1
      while True:
        print "Please select a downstream branch"
        for i, b in enumerate(downstreams):
          print "  %d. %s" % (i, b)
        prompt = "Selection (0-%d)[0]: " % high
        r = opts.pick
        if r:
          print prompt + r
        else:
          r = raw_input(prompt).strip() or '0'
        if not r.isdigit() or (0 > int(r) > high):
          print "Invalid choice."
        else:
          run_stream('checkout', downstreams[int(r)], stdout=sys.stdout,
                     stderr=sys.stderr)
          break
  
  
  if __name__ == '__main__':
    sys.exit(main(sys.argv[1:]))
  
new_contents: #!/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 argparse
  import sys
  
  from git_common import current_branch, branches, upstream, run_stream, hash_one
  
  
  def main(args):
    parser = argparse.ArgumentParser()
    parser.add_argument('--pick',
                        help=(
                            'The number to pick if this command would '
                            'prompt'))
    opts = parser.parse_args(args)
  
    upfn = upstream
    cur = current_branch()
    if cur == 'HEAD':
      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_stream('checkout', downstreams[0], stdout=sys.stdout, stderr=sys.stderr)
    else:
      high = len(downstreams) - 1
      while True:
        print "Please select a downstream branch"
        for i, b in enumerate(downstreams):
          print "  %d. %s" % (i, b)
        prompt = "Selection (0-%d)[0]: " % high
        r = opts.pick
        if r:
          print prompt + r
        else:
          r = raw_input(prompt).strip() or '0'
        if not r.isdigit() or (0 > int(r) > high):
          print "Invalid choice."
        else:
          run_stream('checkout', downstreams[int(r)], stdout=sys.stdout,
                     stderr=sys.stderr)
          break
  
  
  if __name__ == '__main__':
    try:
      sys.exit(main(sys.argv[1:]))
    except KeyboardInterrupt:
      pass
  
subject: Make git-nav-downstream not print a stack if you Ctrl-C it.