Add --merge option to gclient sync.

gclient_scm.py

 # Copyright (c) 2010 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 337 matching lines @@
                      "Please merge or rebase manually:\n" +
                      "cd %s; git rebase %s\n" % (self.checkout_path, new_base) +
                      "OR git checkout -b <some new branch> %s" % new_base)
       raise gclient_utils.Error(switch_error)
     else:
       # case 3 - the default case
       files = self._Capture(['diff', upstream_branch, '--name-only']).split()
       if verbose:
         print('Trying fast-forward merge to branch : %s' % upstream_branch)
       try:
-        merge_output = scm.GIT.Capture(['merge', '--ff-only', upstream_branch],
-            cwd=self.checkout_path)
+        merge_args = ['merge']
+        if not options.merge:
+          merge_args.append('--ff-only')
+        merge_args.append(upstream_branch)
+        merge_output = scm.GIT.Capture(merge_args, cwd=self.checkout_path)
       except gclient_utils.CheckCallError, e:
         if re.match('fatal: Not possible to fast-forward, aborting.', e.stderr):
           if not printed_path:
             print('\n_____ %s%s' % (self.relpath, rev_str))
             printed_path = True
           while True:
             try:
               # TODO(maruel): That can't work with --jobs.
               action = ask_for_data(
                   'Cannot fast-forward merge, attempt to rebase? '
@@ skipping 567 matching lines @@
 
     This method returns a new list to be used as a command."""
     new_command = command[:]
     if revision:
       new_command.extend(['--revision', str(revision).strip()])
     # --force was added to 'svn update' in svn 1.5.
     if ((options.force or options.manually_grab_svn_rev) and
         scm.SVN.AssertVersion("1.5")[0]):
       new_command.append('--force')
     return new_command