Modify scm.GIT.GetUpstreamBranch to behave like git-cl.

trychange.py

 #!/usr/bin/python
 # Copyright (c) 2009 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.
 """Client-side script to send a try job to the try server. It communicates to
 the try server by either writting to a svn repository or by directly connecting
 to the server by HTTP.
 """
 
 import datetime
@@ skipping 195 matching lines @@
 
 class GIT(SCM):
   """Gathers the options and diff for a git checkout."""
   def __init__(self, *args, **kwargs):
     SCM.__init__(self, *args, **kwargs)
     self.checkout_root = scm.GIT.GetCheckoutRoot(self.checkout_root)
     if not self.options.name:
       self.options.name = scm.GIT.GetPatchName(self.checkout_root)
     if not self.options.email:
       self.options.email = scm.GIT.GetEmail(self.checkout_root)
+    if not self.diff_against:
+      self.diff_against = scm.GIT.GetUpstreamBranch(self.checkout_root)
+      if not self.diff_against:
+        print "Unable to determine default branch to diff against."
+        print "Verify this branch is set up to track another"
+        print "(via the --track argument to \"git checkout -b ...\""
     logging.info("GIT(%s)" % self.checkout_root)
 
   def ReadRootFile(self, filename):
     try:
       # A git checkout is always a full checkout.
       data = gclient_utils.FileRead(os.path.join(self.checkout_root, filename))
       logging.debug('%s:\n%s' % (filename, data))
       return data
     except (IOError, OSError):
       logging.debug('%s:\nNone' % filename)
@@ skipping 470 matching lines @@
   except (InvalidScript, NoTryServerAccess), e:
     if swallow_exception:
       return 1
     print e
     return 1
   return 0
 
 
 if __name__ == "__main__":
   sys.exit(TryChange(None, [], False))