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

Previously a `[depot-tools "rebase-update"]` section would show up in the
.git/config on every `git rebase-update` cycle.

R=agable@chromium.org
BUG=

Review URL: https://codereview.chromium.org/228353003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@263423 0039d316-1c4b-4281-b951-d872f2087c98

git_rebase_update.py

@@ -28,7 +28,7 @@ def find_return_branch():
   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)
@@ -244,7 +244,7 @@ def main(args=()):
           % (return_branch, root_branch)
         )
       git.run('checkout', root_branch)
-    git.del_config(STARTING_BRANCH_KEY)
+    git.set_config(STARTING_BRANCH_KEY, '')
 
   return retcode