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

trychange.py

Index: trychange.py
diff --git a/trychange.py b/trychange.py
index 2e5239a9e70fbf84a4d17bf77b1754c1bf429fdc..644a55a915f406a1515f9260b7b35bfdfddaa4d6 100755
--- a/trychange.py
+++ b/trychange.py
@@ -213,6 +213,12 @@ class GIT(SCM):
       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):