git Error: Path is unmerged.

I want to undo some changes on git like this

$ git checkout HEAD foo/bar.txt
error: path 'foo/bar.txt' is unmerged
$ git reset HEAD foo/bar.txt
Unstaged changes after reset:
M       foo/bar.txt

And ended up with the error above.

These commands made my way.

$ git reset foo/bar.txt
$ git checkout foo/bar.txt

BOOM!