遇到這麼一個問題：
You have not concluded your merge (MERGE_HEAD exists). Please, commit
your changes before you can merge.
問題的原因是， 以前的pull 之後merge 失敗， Git 進入conflict 狀態。
解決辦法：
1) Undo the merge and pull again.
To undo a merge:
git merge --abort [Since git version 1.7.4]
git reset --merge [prior git versions]
2) Resolve the conflict.
3) Don’t forget to add and commit the merge.
4) git pull now should work fine.