Make webkit-patch pretty-diff do the right thing for upstream branchs.

sky/tools/webkitpy/common/checkout/scm/git.py

 # Copyright (c) 2009, 2010, 2011 Google Inc. All rights reserved.
 # Copyright (c) 2009 Apple Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
@@ skipping 120 matching lines @@
         return return_code != self.ERROR_FILE_IS_MISSING
 
     def _branch_from_ref(self, ref):
         return ref.replace('refs/heads/', '')
 
     def current_branch(self):
         return self._branch_from_ref(self._run_git(['symbolic-ref', '-q', 'HEAD']).strip())
 
     def _upstream_branch(self):
         current_branch = self.current_branch()
+        # If the remote is pointing to something other than "." than the upstream branch isn't a local branch.
+        if self.read_git_config('branch.%s.remote' % current_branch, cwd=self.checkout_root, executive=self._executive) != ".":
+            return None
         return self._branch_from_ref(self.read_git_config('branch.%s.merge' % current_branch, cwd=self.checkout_root, executive=self._executive).strip())
 
     def _merge_base(self, git_commit=None):
         if git_commit:
             # Rewrite UPSTREAM to the upstream branch
             if 'UPSTREAM' in git_commit:
                 upstream = self._upstream_branch()
                 if not upstream:
                     raise ScriptError(message='No upstream/tracking branch set.')
                 git_commit = git_commit.replace('UPSTREAM', upstream)
 
             # Special-case <refname>.. to include working copy changes, e.g., 'HEAD....' shows only the diffs from HEAD.
             if git_commit.endswith('....'):
                 return git_commit[:-4]
 
             if '..' not in git_commit:
                 git_commit = git_commit + "^.." + git_commit
             return git_commit
 
+        upstream = self._upstream_branch()
+        if upstream:
+            return upstream
+
         return self._remote_merge_base()
 
     def changed_files(self, git_commit=None):
         # FIXME: --diff-filter could be used to avoid the "extract_filenames" step.
         status_command = [self.executable_name, 'diff', '-r', '--name-status', "--no-renames", "--no-ext-diff", "--full-index", self._merge_base(git_commit)]
         # FIXME: I'm not sure we're returning the same set of files that SVN.changed_files is.
         # Added (A), Copied (C), Deleted (D), Modified (M), Renamed (R)
         return self._run_status_and_extract_filenames(status_command, self._status_regexp("ADM"))
 
     def _added_files(self):
@@ skipping 141 matching lines @@
         if self.current_branch() != self._branch_tracking_remote_master():
             return False
         if len(self._local_commits(self._branch_tracking_remote_master())) > 0:
             return False
         return True
 
     def ensure_cleanly_tracking_remote_master(self):
         self._discard_working_directory_changes()
         self._run_git(['checkout', '-q', self._branch_tracking_remote_master()])
         self._discard_local_commits()