Add checks to GitSanityChecks (upstream branch or current branch can be None)

git_cl.py

Index: git_cl.py
diff --git a/git_cl.py b/git_cl.py
index d9b80aa7000ccc98bcc1cd57d6a733c9a2eddc57..39b54d2f5e62b058b9a38f32f5f10a55c84c22a1 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -641,6 +641,15 @@ or verify this branch is set up to track another (via the --track argument to
   def GitSanityChecks(self, upstream_git_obj):
     """Checks git repo status and ensures diff is from local commits."""
 
+    if upstream_git_obj is None:
+      if self.GetBranch() is None:
+        print >> sys.stderr, (
+            'ERROR: unable to dertermine current branch (detached HEAD?)')
+      else:
+        print >> sys.stderr, (
+            'ERROR: no upstream branch')
+      return False
+
     # Verify the commit we're diffing against is in our current branch.
     upstream_sha = RunGit(['rev-parse', '--verify', upstream_git_obj]).strip()
     common_ancestor = RunGit(['merge-base', upstream_sha, 'HEAD']).strip()