Make auto_roll run with a pure git checkout.

tools/push-to-trunk/chromium_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 os
 import sys
 
 from common_includes import *
 
 DEPS_FILE = "DEPS_FILE"
 CHROMIUM = "CHROMIUM"
 
 CONFIG = {
   PERSISTFILE_BASENAME: "/tmp/v8-chromium-roll-tempfile",
   DOT_GIT_LOCATION: ".git",
   DEPS_FILE: "DEPS",
 }
 
 
 class Preparation(Step):
   MESSAGE = "Preparation."
 
   def RunStep(self):
-    self.CommonPrepare()
+    # Update v8 remote tracking branches.
+    self.GitFetchOrigin()
 
 
 class DetectLastPush(Step):
   MESSAGE = "Detect commit ID of last push to trunk."
 
   def RunStep(self):
     self["last_push"] = self._options.last_push or self.FindLastTrunkPush(
-        include_patches=True)
-    self["trunk_revision"] = self.GitSVNFindSVNRev(self["last_push"])
+        branch="origin/master", include_patches=True)
+    self["trunk_revision"] = self.GetCommitPositionNumber(self["last_push"])
     self["push_title"] = self.GitLog(n=1, format="%s",
                                      git_hash=self["last_push"])
 
 
 class SwitchChromium(Step):
   MESSAGE = "Switch to Chromium checkout."
 
   def RunStep(self):
     self["v8_path"] = os.getcwd()
     os.chdir(self._options.chromium)
@@ skipping 101 matching lines @@
       SwitchChromium,
       UpdateChromiumCheckout,
       UploadCL,
       SwitchV8,
       CleanUp,
     ]
 
 
 if __name__ == "__main__":  # pragma: no cover
   sys.exit(ChromiumRoll(CONFIG).Run())