Update .git/objects/info/alternates when upstream url changes.

gclient_scm.py

 # Copyright (c) 2012 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.
 
 """Gclient-specific SCM-specific operations."""
 
 from __future__ import print_function
 
 import errno
 import logging
@@ skipping 434 matching lines @@
         url != 'git://foo' and
         subprocess2.capture(
             ['git', 'config', 'remote.%s.gclient-auto-fix-url' % self.remote],
             cwd=self.checkout_path).strip() != 'False'):
       self.Print('_____ switching %s to a new upstream' % self.relpath)
       if not (options.force or options.reset):
         # Make sure it's clean
         self._CheckClean(rev_str)
       # Switch over to the new upstream
       self._Run(['remote', 'set-url', self.remote, url], options)
+      if mirror:
+        with open(os.path.join(
+            self.checkout_path, '.git', 'objects', 'info', 'alternates'),
+            'w') as fh:
+          fh.write(os.path.join(url, 'objects'))
       self._FetchAndReset(revision, file_list, options)
       return_early = True
 
     if return_early:
       return self._Capture(['rev-parse', '--verify', 'HEAD'])
 
     cur_branch = self._GetCurrentBranch()
 
     # Cases:
     # 0) HEAD is detached. Probably from our initial clone.
@@ skipping 1172 matching lines @@
       new_command.append('--force')
       if command[0] != 'checkout' and scm.SVN.AssertVersion('1.6')[0]:
         new_command.extend(('--accept', 'theirs-conflict'))
     elif options.manually_grab_svn_rev:
       new_command.append('--force')
       if command[0] != 'checkout' and scm.SVN.AssertVersion('1.6')[0]:
         new_command.extend(('--accept', 'postpone'))
     elif command[0] != 'checkout' and scm.SVN.AssertVersion('1.6')[0]:
       new_command.extend(('--accept', 'postpone'))
     return new_command