fatal: refusing to merge unrelated histories：
git pull origin master --allow-unrelated-histories
SSH key problem：
http://stackoverflow.com/questions/12940626/github-error-message-permission-denied-publickey
fatal: refusing to merge unrelated histories：
git pull origin master --allow-unrelated-histories
SSH key problem：
http://stackoverflow.com/questions/12940626/github-error-message-permission-denied-publickey