我正在尝试将我的github分叉存储库与原始存储库同步。我按照github's page上的说明操作了。但是当我尝试将我自己的主分支与上游分开时，它说：
$ git merge upstream/master
error: Your local changes to the following files would be overwritten by merge:
... a list of files ...
Please, commit your changes or stash them before you can merge.
Aborting
当我git commit说nothing to commit (working directory clean)时，就是这种情况。
我错过了什么？