Add --merge option to gclient sync.

BUG=none
TEST=

Review URL: http://codereview.chromium.org/7277035

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@93611 0039d316-1c4b-4281-b951-d872f2087c98

gclient.py

@@ -1090,6 +1090,9 @@ def CMDsync(parser, args):
                          'local modifications')
   parser.add_option('-R', '--reset', action='store_true',
                     help='resets any local changes before updating (git only)')
+  parser.add_option('-M', '--merge', action='store_true',
+                    help='merge upstream changes instead of trying to '
+                         'fast-forward or rebase')
   parser.add_option('--deps', dest='deps_os', metavar='OS_LIST',
                     help='override deps for the specified (comma-separated) '
                          'platform(s); \'all\' will process all deps_os '

gclient_scm.py

@@ -355,8 +355,11 @@ class GitWrapper(SCMWrapper):
       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: