Checkout a git tag with deeply nested submodules

gistfile1.sh

# get a tag
git checkout --force tags/mytag

# from then on I am in a detached head state

# then sync  all submodule to point to the correct rev and force cleaning untracked dir and files
git submodule foreach --recursive 'git submodule sync; git clean -ffd'

# force update to get all submodule content
git submodule update --init --recursive --force

# clean again, forcing untracked file and dir deletion
git clean -ffd