Fix a bug in the GN roll script.

tools/gn/bin/roll_gn.py

 # Copyright 2014 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.
 
 """An auto-roller for GN binaries into Chromium.
 
 This script is used to update the GN binaries that a Chromium
 checkout uses. In order to update the binaries, one must follow
 four steps in order:
 
@@ skipping 358 matching lines @@
       os.remove(desc_file.name)
 
     # Move off of the roll branch onto whatever we were on before.
     # Do not explicitly close the roll CL issue, however; the CQ
     # will close it when the roll lands, assuming it does so.
     self.MoveToLastHead()
 
     return 0
 
   def MovetoLastHead(self):
-    _, out, _ = self.Call('git reflog -1')
-    m = re.match('moving from ([^\s]+)', out)
+    # When this is called, there will be a commit + a checkout as
+    # the two most recent entries in the reflog, assuming nothing as
+    # modified the repo while this script has been running.
+    _, out, _ = self.Call('git reflog -2')
+    m = re.search('moving from ([^\s]+)', out)
     last_head = m.group(1)
     self.Call('git checkout %s' % last_head)
 
   def GetBuildtoolsDesc(self):
     gn_changes = self.GetGNChanges()
     return (
       'Roll gn %s..%s (r%s:r%s)\n'
       '\n'
       '%s'
       '\n'
@@ skipping 40 matching lines @@
   def Call(self, cmd, cwd=None):
     proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, shell=True,
                             cwd=(cwd or self.chromium_src_dir))
     out, err = proc.communicate()
     return proc.returncode, out, err
 
 
 if __name__ == '__main__':
   roller = GNRoller()
   sys.exit(roller.Roll())