fatal: refusing to merge unrelated histories(git pull)

1. https://blog.csdn.net/lindexi_gd/article/details/52554159 (refusing to merge unrelated histories)
2. https://lindexi.gitee.io/post/git-%E9%9C%80%E8%A6%81%E7%9F%A5%E9%81%93%E7%9A%841000%E4%B8%AA%E9%97%AE%E9%A2%98.html(1000 git related problem)