Add --upstream option to gclient to scrape git checkouts back to upstream state.

gclient_scm.py

 # Copyright (c) 2012 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 412 matching lines @@
             continue
           raise
 
       if verbose:
         print(remote_output.strip())
 
       self._UpdateBranchHeads(options, fetch=True)
 
     # This is a big hammer, debatable if it should even be here...
     if options.force or options.reset:
-      self._Run(['reset', '--hard', 'HEAD'], options)
+      target = 'HEAD'
+      if options.upstream and upstream_branch:
+        target = upstream_branch
+      self._Run(['reset', '--hard', target], options)
 
     if current_type == 'detached':
       # case 0
       self._CheckClean(rev_str)
       self._CheckDetachedHead(rev_str, options)
       self._Capture(['checkout', '--quiet', '%s' % revision])
       if not printed_path:
         print('\n_____ %s%s' % (self.relpath, rev_str))
     elif current_type == 'hash':
       # case 1
@@ skipping 127 matching lines @@
     All reverted files will be appended to file_list.
     """
     if not os.path.isdir(self.checkout_path):
       # revert won't work if the directory doesn't exist. It needs to
       # checkout instead.
       print('\n_____ %s is missing, synching instead' % self.relpath)
       # Don't reuse the args.
       return self.update(options, [], file_list)
 
     default_rev = "refs/heads/master"
+    if options.upstream:
+      if self._GetCurrentBranch():
+        upstream_branch = scm.GIT.GetUpstreamBranch(self.checkout_path)
+        default_rev = upstream_branch or default_rev
     _, deps_revision = gclient_utils.SplitUrlRevision(self.url)
     if not deps_revision:
       deps_revision = default_rev
     if deps_revision.startswith('refs/heads/'):
       deps_revision = deps_revision.replace('refs/heads/', 'origin/')
 
     files = self._Capture(['diff', deps_revision, '--name-only']).split()
     self._Run(['reset', '--hard', deps_revision], options)
     self._Run(['clean', '-f', '-d'], options)
     file_list.extend([os.path.join(self.checkout_path, f) for f in files])
@@ skipping 738 matching lines @@
       new_command.append('--force')
       if command[0] != 'checkout' and scm.SVN.AssertVersion('1.6')[0]:
         new_command.extend(('--accept', 'theirs-conflict'))
     elif options.manually_grab_svn_rev:
       new_command.append('--force')
       if command[0] != 'checkout' and scm.SVN.AssertVersion('1.6')[0]:
         new_command.extend(('--accept', 'postpone'))
     elif command[0] != 'checkout' and scm.SVN.AssertVersion('1.6')[0]:
       new_command.extend(('--accept', 'postpone'))
     return new_command