message: git: can_commit function
diff: @@ -90,6 +90,15 @@ class Git(VirtualNode):
  cmd += [revision]
  return self.run(cmd)
  + def can_commit(self):
  + '''Whether there is something ready to commit.'''
  + try:
  + self.run(['diff', '--cached', '--quiet'])
  + except subprocess.CalledProcessError:
  + return True
  + else:
  + return False
  +
  @lru_cache(16)
  def author_date(self):
  """The author date, as given by git %ai format."""
  