[mergeinfo.py] Use remotes/origin/master instead of master

tools/release/test_mergeinfo.py

 #!/usr/bin/env python
 # Copyright 2015 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 mergeinfo
 import shutil
 import unittest
 
 from collections import namedtuple
@@ skipping 12 matching lines @@
 
     fullCommand = ["git", "-C", self.base_dir] + git_args
     p = Popen(args=fullCommand, stdin=PIPE,
         stdout=PIPE, stderr=PIPE)
     output, err = p.communicate()
     rc = p.returncode
     if rc != 0:
       raise Exception(err)
     return output
 
+  def _update_origin(self):
+    # Fetch from origin to get/update the origin/master branch
+    self._execute_git(['fetch', 'origin'])
+
   def setUp(self):
     if path.exists(self.base_dir):
       shutil.rmtree(self.base_dir)
 
     check_call(["git", "init", self.base_dir])
 
+    # Add fake remote with name 'origin'
+    self._execute_git(['remote', 'add', 'origin', self.base_dir])
+
     # Initial commit
     message = '''Initial commit'''
 
     self._make_empty_commit(message)
 
   def tearDown(self):
     if path.exists(self.base_dir):
       shutil.rmtree(self.base_dir)
 
   def _assert_correct_standard_result(
@@ skipping 12 matching lines @@
         result[hash_of_first_commit][0],
         "Child commit wrong")
 
   def _get_commits(self):
     commits = self._execute_git(
         ["log", "--format=%H", "--reverse"]).splitlines()
     return commits
 
   def _make_empty_commit(self, message):
     self._execute_git(["commit", "--allow-empty", "-m", message])
+    self._update_origin()
     return self._get_commits()[-1]
 
   def testCanDescribeCommit(self):
     commits = self._get_commits()
     hash_of_first_commit = commits[0]
 
     result = mergeinfo.describe_commit(
         self.base_dir,
         hash_of_first_commit).splitlines()
 
@@ skipping 90 matching lines @@
       self.base_dir, hash_of_first_commit), 'No Canary coverage')
 
     self._execute_git(['branch', 'remotes/origin/chromium/2345'])
     self._execute_git(['branch', 'remotes/origin/chromium/2346'])
 
     self.assertEqual(mergeinfo.get_first_canary(
       self.base_dir, hash_of_first_commit), '2345')
 
 if __name__ == "__main__":
    unittest.main()