Use --force to reset a git repo before attempting to rebase during gclient sync/update

gclient_scm.py

 # Copyright (c) 2009 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.
 
 """Gclient-specific SCM-specific operations."""
 
 import logging
 import os
 import posixpath
 import re
@@ skipping 195 matching lines @@
                                 '\tAlready in a conflict, i.e. (no branch).\n'
                                 '\tFix the conflict and run gclient again.\n'
                                 '\tOr to abort run:\n\t\tgit-rebase --abort\n'
                                 '\tSee man git-rebase for details.\n'
                                  % (self.relpath, rev_str))
 
     merge_base = self._Run(['merge-base', 'HEAD', new_base])
     self._Run(['remote', 'update'], redirect_stdout=False)
     files = self._Run(['diff', new_base, '--name-only']).split()
     file_list.extend([os.path.join(self.checkout_path, f) for f in files])
+    if options.force:
+      self._Run(['reset', '--hard', merge_base], redirect_stdout=False)
     self._Run(['rebase', '-v', '--onto', new_base, merge_base, cur_branch],
                 redirect_stdout=False, checkrc=False)
 
     # If the rebase generated a conflict, abort and ask user to fix
     if self._GetCurrentBranch() is None:
       raise gclient_utils.Error('\n____ %s%s\n'
                                 '\nConflict while rebasing this branch.\n'
                                 'Fix the conflict and run gclient again.\n'
                                 'See man git-rebase for details.\n'
                                 % (self.relpath, rev_str))
@@ skipping 321 matching lines @@
       print("\n________ couldn't run \'%s\' in \'%s\':\nThe directory "
             "does not exist."
             % (' '.join(command), path))
       # There's no file list to retrieve.
     else:
       self.RunAndGetFileList(options, command, path, file_list)
 
   def FullUrlForRelativeUrl(self, url):
     # Find the forth '/' and strip from there. A bit hackish.
     return '/'.join(self.url.split('/')[:4]) + url