Improve diagnostic message for wrong gclient config

gclient.py

 #!/usr/bin/env python
 # 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.
 
 """Meta checkout manager supporting both Subversion and GIT."""
 # Files
 #   .gclient      : Current client configuration, written by 'config' command.
 #                   Format is a Python script defining 'solutions', a list whose
 #                   entries each are maps binding the strings "name" and "url"
@@ skipping 1215 matching lines @@
 
   def _CheckConfig(self):
     """Verify that the config matches the state of the existing checked-out
     solutions."""
     for dep in self.dependencies:
       if dep.managed and dep.url:
         scm = gclient_scm.CreateSCM(
             dep.url, self.root_dir, dep.name, self.outbuf)
         actual_url = scm.GetActualRemoteURL(self._options)
         if actual_url and not scm.DoesRemoteURLMatch(self._options):
+          mirror = scm.GetCacheMirror()
+          if mirror:
+            mirror_string = '%s (exists=%s)' % (mirror.mirror_path,
+                                                mirror.exists())
+          else:
+            mirror_string = 'not used'
           raise gclient_utils.Error('''
 Your .gclient file seems to be broken. The requested URL is different from what
 is actually checked out in %(checkout_path)s.
 
 The .gclient file contains:
-%(expected_url)s (%(expected_scm)s)
+URL: %(expected_url)s (%(expected_scm)s)
+Cache mirror: %(mirror_string)s
 
 The local checkout in %(checkout_path)s reports:
 %(actual_url)s (%(actual_scm)s)
 
 You should ensure that the URL listed in .gclient is correct and either change
 it or fix the checkout. If you're managing your own git checkout in
 %(checkout_path)s but the URL in .gclient is for an svn repository, you probably
 want to set 'managed': False in .gclient.
 '''  % {'checkout_path': os.path.join(self.root_dir, dep.name),
         'expected_url': dep.url,
         'expected_scm': gclient_scm.GetScmName(dep.url),
+        'mirror_string' : mirror_string,
         'actual_url': actual_url,
         'actual_scm': gclient_scm.GetScmName(actual_url)})
 
   def SetConfig(self, content):
     assert not self.dependencies
     config_dict = {}
     self.config_content = content
     try:
       exec(content, config_dict)
     except SyntaxError as e:
@@ skipping 1079 matching lines @@
 
 
 if '__main__' == __name__:
   try:
     sys.exit(main(sys.argv[1:]))
   except KeyboardInterrupt:
     sys.stderr.write('interrupted\n')
     sys.exit(1)
 
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