Fix auto roll update.

tools/push-to-trunk/auto_roll.py

 #!/usr/bin/env python
 # Copyright 2014 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import argparse
 import json
 import os
 import sys
 import urllib
@@ skipping 24 matching lines @@
     result = self.ReadURL(search_url, params, wait_plan=[5, 20])
     if self.ContainsChromiumRoll(json.loads(result)["results"]):
       print "Stop due to existing Chromium roll."
       return True
 
 
 class DetectLastPush(Step):
   MESSAGE = "Detect commit ID of the last push to trunk."
 
   def RunStep(self):
+    self.vc.Fetch()
     push_hash = self.FindLastTrunkPush(
         branch="origin/candidates", include_patches=True)
     self["last_push"] = self.GetCommitPositionNumber(push_hash)
 
 
 class DetectLastRoll(Step):
   MESSAGE = "Detect commit ID of the last Chromium roll."
 
   def RunStep(self):
     # Interpret the DEPS file to retrieve the v8 revision.
@@ skipping 77 matching lines @@
       CheckActiveRoll,
       DetectLastPush,
       DetectLastRoll,
       CheckClusterFuzz,
       RollChromium,
     ]
 
 
 if __name__ == "__main__":  # pragma: no cover
   sys.exit(AutoRoll().Run())