Make marked merge base invalid when the upstream changes.

git_mark_merge_base.py

 #!/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.
 
 """
 Explicitly set/remove/print the merge-base for the current branch.
 
 This manually set merge base will be a stand-in for `git merge-base` for the
 purposes of the chromium depot_tools git extensions. Passing no arguments will
 just print the effective merge base for the current branch.
 """
 
 import argparse
 import sys
 
 from subprocess2 import CalledProcessError
 
 from git_common import remove_merge_base, manual_merge_base, current_branch
-from git_common import get_or_create_merge_base, hash_one
+from git_common import get_or_create_merge_base, hash_one, upstream
 
 
 def main(argv):
   parser = argparse.ArgumentParser(
     description=__doc__.strip().splitlines()[0],
     epilog=' '.join(__doc__.strip().splitlines()[1:]))
   g = parser.add_mutually_exclusive_group()
   g.add_argument(
     'merge_base', nargs='?',
     help='The new hash to use as the merge base for the current branch'
@@ skipping 13 matching lines @@
 
   if opts.merge_base:
     try:
       opts.merge_base = hash_one(opts.merge_base)
     except CalledProcessError:
       print >> sys.stderr, (
           'fatal: could not resolve %s as a commit' % (opts.merge_base)
       )
       return 1
 
-    manual_merge_base(cur, opts.merge_base)
+    manual_merge_base(cur, opts.merge_base, upstream(cur))
 
   ret = 0
   actual = get_or_create_merge_base(cur)
   if opts.merge_base and opts.merge_base != actual:
     ret = 1
     print "Invalid merge_base %s" % opts.merge_base
 
   print "merge_base(%s): %s" % (cur, actual)
 
   return ret
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv[1:]))