function TREE({ ref = 'HEAD' } = {}) {
  const o = Object.create(null)
  Object.defineProperty(o, GitWalkSymbol, {
    value: function({ fs, gitdir, cache }) {
      return new GitWalkerRepo({ fs, gitdir, ref, cache })
    },
  })
  Object.freeze(o)
  return o
}
@param {object} args
@param {string} [args.ref='HEAD']
@returns {Walker}