input: Write a python function to retrieve the commit message of the last commit using os.popen.
  assert get_last_commit_message() is not None  # Ensure the function returns a non-None value
  assert isinstance(get_last_commit_message(), str)  # Ensure the function returns a string
output: def get_last_commit_message():
      import os
      commit_message = os.popen('git log -1 --pretty=%B').read().strip()
      return commit_message