Correctly guard file_list modification in gclient update

gclient_scm.py

 # 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.
 
 """Gclient-specific SCM-specific operations."""
 
 from __future__ import print_function
 
 import errno
 import logging
@@ skipping 656 matching lines @@
           if not printed_path:
             self.Print('_____ %s at %s' % (self.relpath, revision),
                        timestamp=False)
             printed_path = True
           self.Print(merge_output.strip())
           if not verbose:
             # Make the output a little prettier. It's nice to have some
             # whitespace between projects when syncing.
             self.Print('')
 
-      file_list.extend(
-          [os.path.join(self.checkout_path, f) for f in rebase_files])
+      if file_list is not None:
+        file_list.extend(
+            [os.path.join(self.checkout_path, f) for f in rebase_files])
 
     # If the rebase generated a conflict, abort and ask user to fix
     if self._IsRebasing():
       raise gclient_utils.Error('\n____ %s at %s\n'
                                 '\nConflict while rebasing this branch.\n'
                                 'Fix the conflict and run gclient again.\n'
                                 'See man git-rebase for details.\n'
                                 % (self.relpath, revision))
 
     if verbose:
@@ skipping 485 matching lines @@
     kwargs.setdefault('cwd', self.checkout_path)
     kwargs.setdefault('stdout', self.out_fh)
     kwargs['filter_fn'] = self.filter
     kwargs.setdefault('print_stdout', False)
     env = scm.GIT.ApplyEnvVars(kwargs)
     cmd = ['git'] + args
     if show_header:
       gclient_utils.CheckCallAndFilterAndHeader(cmd, env=env, **kwargs)
     else:
       gclient_utils.CheckCallAndFilter(cmd, env=env, **kwargs)