error: merge is not possible because you have unmerged files.

hint: Fix them up in the work tree, and then use 'git add/rm '

hint: as appropriate to mark resolution and make a commit.

fatal: Exiting because of an unresolved conflict.

執行

git add .

git commit -a -m "add ."

git pull  origin master

git merge origin/master

成功解決!

參考資料：

https://stackoverflow.com/questions/21868857/removing-xcuserstate-and-ds-store-files-from-git

http://www.jianshu.com/p/4f69c79b295f