Fix empty section appearing in git config for every rebase-update.

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 10 matching lines @@
 STARTING_BRANCH_KEY = 'depot-tools.rebase-update.starting-branch'
 
 
 def find_return_branch():
   """Finds the branch which we should return to after rebase-update completes.
 
   This value may persist across multiple invocations of rebase-update, if
   rebase-update runs into a conflict mid-way.
   """
   return_branch = git.config(STARTING_BRANCH_KEY)
-  if return_branch is None:
+  if not return_branch:
     return_branch = git.current_branch()
     if return_branch != 'HEAD':
       git.set_config(STARTING_BRANCH_KEY, return_branch)
 
   return return_branch
 
 
 def fetch_remotes(branch_tree):
   """Fetches all remotes which are needed to update |branch_tree|."""
   fetch_tags = False
@@ skipping 195 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)
-    git.del_config(STARTING_BRANCH_KEY)
+    git.set_config(STARTING_BRANCH_KEY, '')
 
   return retcode
 
 
 if __name__ == '__main__':  # pragma: no cover
   sys.exit(main(sys.argv[1:]))