Added message when upstream branch is gone.

git_cl.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.
 
 # Copyright (C) 2008 Evan Martin <martine@danga.com>
 
 """A git-command for integrating reviews on Rietveld."""
 
 from distutils.version import LooseVersion
@@ skipping 120 matching lines @@
   return RunGitWithCode(args, suppress_stderr=True)[1]
 
 
 def IsGitVersionAtLeast(min_version):
   prefix = 'git version '
   version = RunGit(['--version']).strip()
   return (version.startswith(prefix) and
       LooseVersion(version[len(prefix):]) >= LooseVersion(min_version))
 
 
+def BranchExists(branch):
+  """Return True if specified branch exists."""
+  code, _ = RunGitWithCode(['rev-parse', '--verify', branch],
+                           suppress_stderr=True)
+  return not code
+
+
 def ask_for_data(prompt):
   try:
     return raw_input(prompt)
   except KeyboardInterrupt:
     # Hide the exception.
     sys.exit(1)
 
 
 def git_set_branch_value(key, value):
   branch = Changelist().GetBranch()
@@ skipping 560 matching lines @@
           else:
             DieWithError("""Unable to determine default branch to diff against.
 Either pass complete "git diff"-style arguments, like
   git cl upload origin/master
 or verify this branch is set up to track another (via the --track argument to
 "git checkout -b ...").""")
 
     return remote, upstream_branch
 
   def GetCommonAncestorWithUpstream(self):
+    upstream_branch = self.GetUpstreamBranch()
+    if not BranchExists(upstream_branch):
+      DieWithError('The upstream for the current branch (%s) does not exist '
+                   'anymore.\nPlease fix it and try again.' % self.GetBranch())
     return git_common.get_or_create_merge_base(self.GetBranch(),
-                                               self.GetUpstreamBranch())
+                                               upstream_branch)
 
   def GetUpstreamBranch(self):
     if self.upstream_branch is None:
       remote, upstream_branch = self.FetchUpstreamTuple(self.GetBranch())
       if remote is not '.':
         upstream_branch = upstream_branch.replace('refs/heads/',
                                                   'refs/remotes/%s/' % remote)
         upstream_branch = upstream_branch.replace('refs/branch-heads/',
                                                   'refs/remotes/branch-heads/')
       self.upstream_branch = upstream_branch
@@ skipping 2639 matching lines @@
 if __name__ == '__main__':
   # These affect sys.stdout so do it outside of main() to simplify mocks in
   # unit testing.
   fix_encoding.fix_encoding()
   colorama.init()
   try:
     sys.exit(main(sys.argv[1:]))
   except KeyboardInterrupt:
     sys.stderr.write('interrupted\n')
     sys.exit(1)