Make git-rebase-update a bit more robust

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
 import collections
 import logging
 import sys
 import textwrap
+import os
 
 from fnmatch import fnmatch
 from pprint import pformat
 
 import git_common as git
 
 
 STARTING_BRANCH_KEY = 'depot-tools.rebase-update.starting-branch'
+STARTING_WORKDIR_KEY = 'depot-tools.rebase-update.starting-workdir'
 
 
-def find_return_branch():
-  """Finds the branch which we should return to after rebase-update completes.
+def find_return_branch_workdir():
+  """Finds the branch and working directory which we should return to after
+  rebase-update completes.
 
-  This value may persist across multiple invocations of rebase-update, if
+  These values may persist across multiple invocations of rebase-update, if
   rebase-update runs into a conflict mid-way.
   """
   return_branch = git.config(STARTING_BRANCH_KEY)
+  workdir = git.config(STARTING_WORKDIR_KEY)
   if not return_branch:
+    workdir = os.getcwd()
+    git.set_config(STARTING_WORKDIR_KEY, workdir)
     return_branch = git.current_branch()
     if return_branch != 'HEAD':
       git.set_config(STARTING_BRANCH_KEY, return_branch)
 
-  return return_branch
+  return return_branch, workdir
 
 
 def fetch_remotes(branch_tree):
   """Fetches all remotes which are needed to update |branch_tree|."""
   fetch_tags = False
   remotes = set()
   tag_set = git.tags()
   fetchspec_map = {}
   all_fetchspec_configs = git.run(
       'config', '--get-regexp', r'^remote\..*\.fetch').strip()
@@ skipping 159 matching lines @@
 
   if git.in_rebase():
     # TODO(iannucci): Be able to resume rebase with flags like --continue,
     # etc.
     print (
       'Rebase in progress. Please complete the rebase before running '
       '`git rebase-update`.'
     )
     return 1
 
-  return_branch = find_return_branch()
+  return_branch, return_workdir = find_return_branch_workdir()
+  os.chdir(git.run('rev-parse', '--show-toplevel'))
 
   if git.current_branch() == 'HEAD':
     if git.run('status', '--porcelain'):
       print 'Cannot rebase-update with detached head + uncommitted changes.'
       return 1
   else:
     git.freeze()  # just in case there are any local changes.
 
   skipped, branch_tree = git.get_branch_tree()
   for branch in skipped:
@@ skipping 29 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)
     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)