After a bit of googling and going through the mailing lists, I finally found the solution:
1. Go to ./repo/manifests
2. $ git reset --hard
3. $ repo sync
[Please note that there are two dashes "- -" before 'hard']
repo撤销本地修改
repo forall -c git reset --hard HEAD
有的时候修改本地代码了，再次同步的时候会报错，需要清理代码的时候，可以执行这个命令