refusing to merge unrelated histories
如果git merge合并的时候出现refusing to merge unrelated histories的错误，原因是两个仓库不同而导致的，需要在后面加上--allow-unrelated-histories进行允许合并，即可解决问题。
git push origin <branch_name>
Updates were rejected because the tip of your current branch is behind
使用git branch --set-upstream-to=origin/master master将本地关联到远程，在git push。