Make git-rebase-update a bit more robust

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 507 matching lines @@
               rebase.
   """
   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')
+      run_with_retcode('rebase', '--abort')  # ignore failure
     return RebaseRet(False, cpe.stdout, cpe.stderr)
 
 
 def remove_merge_base(branch):
   del_branch_config(branch, 'base')
   del_branch_config(branch, 'base-upstream')
 
 
 def root():
   return config('depot-tools.upstream', 'origin/master')
@@ skipping 248 matching lines @@
         hash=branch_hash, upstream=upstream_branch, ahead=ahead, behind=behind)
 
   # Set None for upstreams which are not branches (e.g empty upstream, remotes
   # and deleted upstream branches).
   missing_upstreams = {}
   for info in info_map.values():
     if info.upstream not in info_map and info.upstream not in missing_upstreams:
       missing_upstreams[info.upstream] = None
 
   return dict(info_map.items() + missing_upstreams.items())