Fix ancestor check

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 419 matching lines @@
     return None
   actual_merge_base = run('merge-base', parent, branch)
 
   if base_upstream != parent:
     base = None
     base_upstream = None
 
   def is_ancestor(a, b):
     return run_with_retcode('merge-base', '--is-ancestor', a, b) == 0
 
-  if base:
+  if base and base != actual_merge_base:
     if not is_ancestor(base, branch):
       logging.debug('Found WRONG pre-set merge-base for %s: %s', branch, base)
       base = None
     elif is_ancestor(base, actual_merge_base):
       logging.debug('Found OLD pre-set merge-base for %s: %s', branch, base)
       base = None
     else:
       logging.debug('Found pre-set merge-base for %s: %s', branch, base)
 
   if not base:
@@ skipping 453 matching lines @@
                       ['HEAD'])
 
 
 def clone_file(repository, new_workdir, link, operation):
   if not os.path.exists(os.path.join(repository, link)):
     return
   link_dir = os.path.dirname(os.path.join(new_workdir, link))
   if not os.path.exists(link_dir):
     os.makedirs(link_dir)
   operation(os.path.join(repository, link), os.path.join(new_workdir, link))