Only chdir in rebase-update if there's a directory to go back to.

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 253 matching lines @@
       git.run('checkout', return_branch)
       git.thaw()
     else:
       root_branch = git.root()
       if return_branch != 'HEAD':
         print (
           "%r was merged with its parent, checking out %r instead."
           % (return_branch, root_branch)
         )
       git.run('checkout', root_branch)
-    os.chdir(return_workdir)
+    if return_workdir:
+      os.chdir(return_workdir)
     git.set_config(STARTING_BRANCH_KEY, '')
     git.set_config(STARTING_WORKDIR_KEY, '')
 
   return retcode
 
 
 if __name__ == '__main__':  # pragma: no cover
   try:
     sys.exit(main())
   except KeyboardInterrupt:
     sys.stderr.write('interrupted\n')
     sys.exit(1)