code: def _get_subdirectory(cls, location):
          
          # find the repo root
          git_dir = cls.run_command(['rev-parse', '--git-dir'],
                                    show_stdout=False, cwd=location).strip()
          if not os.path.isabs(git_dir):
              git_dir = os.path.join(location, git_dir)
          root_dir = os.path.join(git_dir, '..')
          # find setup.py
          orig_location = location
          while not os.path.exists(os.path.join(location, 'setup.py')):
              last_location = location
              location = os.path.dirname(location)
              if location == last_location:
                  # We've traversed up to the root of the filesystem without
                  # finding setup.py
                  logger.warning(
                      "Could not find setup.py for directory %s (tried all "
                      "parent directories)",
                      orig_location,
                  )
                  return None
          # relative path of setup.py to repo root
          if samefile(root_dir, location):
              return None
          return os.path.relpath(location, root_dir)
docs: Return the relative path of setup.py to the git repo root.