Avoid computing patch_root in get_files_affected_by_patch.

recipe_modules/tryserver/api.py

 # Copyright 2014 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.
 
 import contextlib
 import hashlib
 
 from recipe_engine import recipe_api
 
 
@@ skipping 131 matching lines @@
           authentication=authentication)
     elif storage == PATCH_STORAGE_SVN:
       return self.apply_from_svn(cwd)
     elif storage == PATCH_STORAGE_GIT:
       return self.apply_from_git(cwd)
     else:
       # Since this method is "maybe", we don't raise an Exception.
       pass
 
   def get_files_affected_by_patch(self):
+    # DO NOT USE THIS. Use same method in gclient recipe_module.
+    # TODO(tandrii): remove this method completely.
     git_diff_kwargs = {}
     issue_root = self.m.rietveld.calculate_issue_root()
     if issue_root:
       git_diff_kwargs['cwd'] = self.m.path['checkout'].join(issue_root)
     step_result = self.m.git('diff', '--cached', '--name-only',
                              name='git diff to analyze patch',
                              stdout=self.m.raw_io.output(),
                              step_test_data=lambda:
                                self.m.raw_io.test_api.stream_output('foo.cc'),
                              **git_diff_kwargs)
@@ skipping 76 matching lines @@
       self.add_failure_reason(e.reason)
 
       failure_hash = hashlib.sha1()
       failure_hash.update(self.m.json.dumps(self._failure_reasons))
 
       step_result = self.m.step.active_result
       step_result.presentation.properties['failure_hash'] = \
           failure_hash.hexdigest()
 
       raise