git push 时候遇到的错误：
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Integrate the remote changes (e.g.
hint: ‘git pull …’) before pushing again.
hint: See the ‘Note about fast-forwards’ in ‘git push –help’ for details.
错误原因：
两个不同的项目需要先合并。
解决办法：
git pull origin master –allow-unrelated-histories