遇到这么一个问题：

You have not concluded your merge (MERGE_HEAD exists). Please, commit
your changes before you can merge.

问题的原因是， 以前的pull 之后merge 失败， git 进入conflict 状态。
解决办法：

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.