Make mark-merge-base cap the marked merge base at the real merge-base.

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 227 matching lines @@
 
 
 def branches(*args):
   NO_BRANCH = ('* (no branch', '* (detached from ')
   for line in run('branch', *args).splitlines():
     if line.startswith(NO_BRANCH):
       continue
     yield line.split()[-1]
 
 
+def run_with_retcode(*cmd, **kwargs):
+  """Run a command but only return the status code."""
+  try:
+    run(*cmd, **kwargs)
+    return 0
+  except subprocess2.CalledProcessError as cpe:
+    return cpe.returncode
+
+
 def config(option, default=None):
   try:
     return run('config', '--get', option) or default
   except subprocess2.CalledProcessError:
     return default
 
 
 def config_list(option):
   try:
     return run('config', '--get-all', option).split()
@@ skipping 57 matching lines @@
 
   return skipped, branch_tree
 
 
 def get_or_create_merge_base(branch, parent=None):
   """Finds the configured merge base for branch.
 
   If parent is supplied, it's used instead of calling upstream(branch).
   """
   base = branch_config(branch, 'base')
+  parent = parent or upstream(branch)
+  actual_merge_base = run('merge-base', parent, branch)
+
+  def is_ancestor(a, b):
+    return run_with_retcode('merge-base', '--is-ancestor', a, b) == 0
+
   if base:
-    try:
-      run('merge-base', '--is-ancestor', base, branch)
-      logging.debug('Found pre-set merge-base for %s: %s', branch, base)
-    except subprocess2.CalledProcessError:
+    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:
-    base = run('merge-base', parent or upstream(branch), branch)
+    base = actual_merge_base
     manual_merge_base(branch, base)
 
   return base
 
 
 def hash_multi(*reflike):
   return run('rev-parse', *reflike).splitlines()
 
 
 def hash_one(reflike):
@@ skipping 267 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