tryserver: make it possible to override cwd for 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 collections
 import contextlib
 import hashlib
 
 from recipe_engine import recipe_api
 
@@ skipping 131 matching lines @@
           self.m.rietveld.calculate_issue_root(),
           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, patch_root=None):
+  def get_files_affected_by_patch(self, patch_root=None, **kwargs):
     """Returns list of paths to files affected by the patch.
 
     Argument:
       patch_root: path relative to api.path['root'], usually obtained from
         api.gclient.calculate_patch_root(patch_project)
 
     Returned paths will be relative to to patch_root.
 
     TODO(tandrii): remove this doc.
     Unless you use patch_root=None, in which case old behavior is used
     which returns paths relative to checkout aka solution[0].name.
     """
     # patch_root must be set! None is for backwards compataibility and will be
     # removed.
     if patch_root is None:
       return self._old_get_files_affected_by_patch()
+    if not kwargs.get('cwd'):
+      kwargs['cwd'] = self.m.path['slave_build'].join(patch_root)
     step_result = self.m.git('diff', '--cached', '--name-only',
-                             cwd=self.m.path['slave_build'].join(patch_root),
                              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'))
+                               self.m.raw_io.test_api.stream_output('foo.cc'),
+                             **kwargs)
     paths = [self.m.path.join(patch_root, p) for p in
              step_result.stdout.split()]
     if self.m.platform.is_win:
       # Looks like "analyze" wants POSIX slashes even on Windows (since git
       # uses that format even on Windows).
       paths = [path.replace('\\', '/') for path in paths]
     step_result.presentation.logs['files'] = paths
     return paths
 
 
@@ skipping 117 matching lines @@
     result = self.m.python(
         'parse description', self.package_repo_resource('git_footers.py'),
         args=['--json', self.m.json.output()],
         stdin=self.m.raw_io.input(data=patch_text))
     return result.json.output
 
   def get_footer(self, tag, patch_text=None):
     """Gets a specific tag from a CL description"""
     return self.get_footers(patch_text).get(tag, [])