How do I discard unstaged changes in Git?

It seems like the complete solution is:

git clean -df
git checkout -- .

WARNING: while it won't delete ignored files mentioned directly in .gitignore, git clean -df may delete ignored files residing in folders.

git clean removes all untracked files and git checkout clears all unstaged changes.

This checks out the current index for the current directory, throwing away all changes in files from the current directory downwards.

git checkout .

or this which checks out all files from the index, overwriting working tree files.

git checkout-index -a -f