Support for |change| argument to |GetPreferredTrySlaves()|, try 2.

git_try.py

 #!/usr/bin/python
 # Copyright (c) 2011 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.
 """Wrapper for trychange.py for git checkout."""
 
 import logging
 import sys
 
 import breakpad  # pylint: disable=W0611
 
 from scm import GIT
 import subprocess2
 import third_party.upload
 import trychange
+import git_cl
 
 
 def GetRietveldIssueNumber():
   try:
     return GIT.Capture(
         ['config', 'branch.%s.rietveldissue' % GIT.GetBranch(None)])
   except subprocess2.CalledProcessError:
     return None
 
 
@@ skipping 20 matching lines @@
         '--issue', GetRietveldIssueNumber(),
         '--patchset', patchset,
     ])
   else:
     rietveld_url = GetRietveldServerUrl()
     if rietveld_url:
       args.extend(['--rietveld_url', GetRietveldServerUrl()])
   # Hack around a limitation in logging.
   logging.getLogger().handlers = []
   try:
+    cl = git_cl.Changelist()
+    change = cl.GetChange(cl.GetUpstreamBranch(), None)
     sys.exit(trychange.TryChange(
-        args, file_list=[], swallow_exception=False,
+        args, change, file_list=[], swallow_exception=False,
         prog='git try',
         extra_epilog='\n'
                      'git try will diff against your tracked branch and will '
                      'detect your rietveld\n'
                      'code review if you are using git-cl\n'))
   except third_party.upload.ClientLoginError, e:
     print('Got an exception while trying to log in to Rietveld.')
     print(str(e))