[recipe_modules/step] do not set cwd if it is start_dir.

recipe_modules/step/api.py

 # Copyright 2013 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 import contextlib
 import copy
 
 from recipe_engine import recipe_api
 
 
@@ skipping 135 matching lines @@
       ok_ret (tuple or set of ints, str): allowed return codes. Any unexpected
         return codes will cause an exception to be thrown. If you pass in the
         value 'any' or 'all', the engine will allow any return code to be
         returned. Defaults to {0}
       infra_step: Whether or not this is an infrastructure step. Infrastructure
         steps will place the step in an EXCEPTION state and raise InfraFailure.
       wrapper: If supplied, a command to prepend to the executed step as a
         command wrapper.
       timeout: If supplied, the recipe engine will kill the step after the
         specified number of seconds.
-      cwd (str or None): absolute path to working directory for the command
       allow_subannotations (bool): if True, lets the step emit its own
           annotations. NOTE: Enabling this can cause some buggy behavior. Please
           strongly consider using step_result.presentation instead. If you have
           questions, please contact infra-dev@chromium.org.
       trigger_specs: a list of trigger specifications
       stdout: Placeholder to put step stdout into. If used, stdout won't appear
           in annotator's stdout (and |allow_subannotations| is ignored).
       stderr: Placeholder to put step stderr into. If used, stderr won't appear
           in annotator's stderr.
       stdin: Placeholder to read step stdin from.
@@ skipping 41 matching lines @@
     while True:
       full_name = self.combine_with_context('name', name)
       if full_name not in self._seen_steps:
         break
 
       step_count = self._step_names.setdefault(full_name, 1) + 1
       self._step_names[full_name] = step_count
       name = "%s (%d)" % (name, step_count)
     self._seen_steps.add(full_name)
 
-    if 'cwd' not in kwargs:
-      kwargs['cwd'] = self.get_from_context('cwd')
+    cwd = self.get_from_context('cwd')
+    if cwd is not None and cwd != self.m.path['start_dir']:
+      kwargs['cwd'] = cwd
     kwargs['env'] = self.get_from_context('env', {})
     if self._prefix_path:
       ps = self.m.path.pathsep
       prefix = ps.join(self._prefix_path)
       suffix = kwargs['env'].get('PATH', '%(PATH)s')
       kwargs['env']['PATH'] = '%s%s%s' % (prefix, ps, suffix)
     kwargs['infra_step'] = self.combine_with_context(
         'infra_step', bool(infra_step))
     kwargs['step_nest_level'] = self.combine_with_context('nest_level', 0)
     kwargs['name'] = full_name
     kwargs['base_name'] = name
 
     schema = self.make_config()
     schema.set_val(kwargs)
     return self.run_from_dict(schema.as_jsonish())
 
   # TODO(martiniss) delete, and make generator_script use **kwargs on step()
   @recipe_api.composite_step
   def run_from_dict(self, dct):
     return self.step_client.run_step(dct)