Avoid computing patch_root in get_files_affected_by_patch.

recipe_modules/tryserver/api.py

Index: recipe_modules/tryserver/api.py
diff --git a/recipe_modules/tryserver/api.py b/recipe_modules/tryserver/api.py
index 8bd8df87f94bdfb68e7ad7f91ee0a15f54fec61a..e2088afe8fbfa03858a1e860430cb57efe4d8d22 100644
--- a/recipe_modules/tryserver/api.py
+++ b/recipe_modules/tryserver/api.py
@@ -149,6 +149,8 @@ class TryserverApi(recipe_api.RecipeApi):
       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: