Fix the print of the rebase error message, and make the output prettier too.

git_common.py

 # 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.
 
 # Monkeypatch IMapIterator so that Ctrl-C can kill everything properly.
 # Derived from https://gist.github.com/aljungberg/626518
 import multiprocessing.pool
 from multiprocessing.pool import IMapIterator
 def wrapper(func):
   def wrap(self, timeout=None):
@@ skipping 430 matching lines @@
   """
   try:
     args = ['--onto', parent, start, branch]
     if TEST_MODE:
       args.insert(0, '--committer-date-is-author-date')
     run('rebase', *args)
     return RebaseRet(True, '')
   except subprocess2.CalledProcessError as cpe:
     if abort:
       run('rebase', '--abort')
-    return RebaseRet(False, cpe.output)
+    return RebaseRet(False, cpe.stdout)
 
 
 def remove_merge_base(branch):
   del_branch_config(branch, 'base')
 
 
 def root():
   return config('depot-tools.upstream', 'origin/master')
 
 
@@ skipping 166 matching lines @@
     return None
   return ret
 
 
 def upstream(branch):
   try:
     return run('rev-parse', '--abbrev-ref', '--symbolic-full-name',
                branch+'@{upstream}')
   except subprocess2.CalledProcessError:
     return None