Improve error message when the upstream branch is wrong.

git_cl.py

 #!/usr/bin/env 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.
 
 # Copyright (C) 2008 Evan Martin <martine@danga.com>
 
 """A git-command for integrating reviews on Rietveld."""
 
 import logging
@@ skipping 477 matching lines @@
       self.SetPatchset(0)
     self.has_issue = False
 
   def GetChange(self, upstream_branch, author):
     root = RunCommand(['git', 'rev-parse', '--show-cdup']).strip() or '.'
     absroot = os.path.abspath(root)
 
     # We use the sha1 of HEAD as a name of this change.
     name = RunCommand(['git', 'rev-parse', 'HEAD']).strip()
     # Need to pass a relative path for msysgit.
-    files = scm.GIT.CaptureStatus([root], upstream_branch)
+    try:
+      files = scm.GIT.CaptureStatus([root], upstream_branch)
+    except subprocess2.CalledProcessError:
+      DieWithError(
+          ('\nFailed to diff against upstream branch %s!\n\n'
+           'This branch probably doesn\'t exist anymore. To reset the\n'
+           'tracking branch, please run\n'
+           '    git branch --set-upstream %s trunk\n'
+           'replacing trunk with origin/master or the relevant branch') %
+          (upstream_branch, self.GetBranch()))
 
     issue = ConvertToInteger(self.GetIssue())
     patchset = ConvertToInteger(self.GetPatchset())
     if issue:
       description = self.GetDescription()
     else:
       # If the change was never uploaded, use the log messages of all commits
       # up to the branch point, as git cl upload will prefill the description
       # with these log messages.
       description = RunCommand(['git', 'log', '--pretty=format:%s%n%n%b',
@@ skipping 922 matching lines @@
              'and retry or visit go/isgaeup.\n%s') % (e.code, str(e)))
 
   # Not a known command. Default to help.
   GenUsage(parser, 'help')
   return CMDhelp(parser, argv)
 
 
 if __name__ == '__main__':
   fix_encoding.fix_encoding()
   sys.exit(main(sys.argv[1:]))