Easy way to delete unused local branches in Git

If you use a new feature branch for every new feature you develop,
you'll end up with a lot of unused local branches.
The laziest way to delete all unused branches safely except master:

git checkout master
git branch | xargs git branch -d

This is safe, because git branch -d only deletes branches that have been merged into another branch,
and it won't delete the current branch, in this case master.

But it's lazy,
because it will produce error messages for every branch that cannot be deleted,
and for the * master line in the output of git branch:

error: The branch 'cool-feature' is not fully merged.
error: branch '*' not found.
error: Cannot delete the branch 'master' which you are currently on.

If you want to be more accurate,
you can exclude branches that have not been merged into master:

git checkout master
git branch --merged master | xargs git branch -d

And if you want to be really accurate,
then you can exclude master too:

git checkout master
git branch --merged master | grep -v ^\* | xargs git branch -d