Change git-rebase-update to always keep branch 'master'.

git_rebase_update.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.
 
 """
 Tool to update all branches to have the latest changes from their upstreams.
 """
 
 import argparse
@@ skipping 66 matching lines @@
                         stderr=sys.stderr)
 
 
 def remove_empty_branches(branch_tree):
   tag_set = git.tags()
   ensure_root_checkout = git.once(lambda: git.run('checkout', git.root()))
 
   deletions = set()
   downstreams = collections.defaultdict(list)
   for branch, parent in git.topo_iter(branch_tree, top_down=False):
+    if branch == 'master':
+      continue
     downstreams[parent].append(branch)
 
     if git.hash_one(branch) == git.hash_one(parent):
       ensure_root_checkout()
 
       logging.debug('branch %s merged to %s', branch, parent)
 
       for down in downstreams[branch]:
         if down in deletions:
           continue
@@ skipping 167 matching lines @@
           % (return_branch, root_branch)
         )
       git.run('checkout', root_branch)
     git.set_config(STARTING_BRANCH_KEY, '')
 
   return retcode
 
 
 if __name__ == '__main__':  # pragma: no cover
   sys.exit(main(sys.argv[1:]))