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
diff --git a/gclient_scm.py b/gclient_scm.py
index 83955c6..9881c07 100644
--- a/gclient_scm.py
+++ b/gclient_scm.py
@@ -355,8 +355,11 @@
       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: