Git: can't undo local changes (error: path ... is unmerged)

You did it the wrong way around. You are meant to reset first, to unstage the file, then checkout, to revert local changes.

Try this:

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

This worked perfectly for me:

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

git checkout origin/[branch] .
git status

// Note dot (.) at the end. And all will be good