# Simple
git filter-branch --prune-empty --subdirectory-filter foobar/ --tag-name-filter cat -- --all

# Complex
# (Ref.: http://stackoverflow.com/a/17867910/11895,
#        http://stackoverflow.com/a/19957874/11895)
git filter-branch -f --prune-empty --index-filter \
  'git rm --cached --ignore-unmatch -r -q -- . ; \
   git reset -q $GIT_COMMIT -- .gitignore README.md foo/ bar/' \
  -- --all