Moving a git repository to the subdirectory of another repository

How do I move a git repository to be the subdirectory of another repository? is a question answered umpteen times, and all answers vary somewhat. For my needs, this suffices:

#!/bin/bash

case $# in
  3 ) old_repo=$1 new_repo=$2 new_name=$3 ;;
  * ) echo "$0: old_repo new_repo new_name" ; exit 2 ;;
esac

set -e

trap 'rm -rf $old_work $new_work' EXIT
old_work=$(mktemp -d)
new_work=$(mktemp -d)
git clone $old_repo $old_work
git clone $new_repo $new_work

cd $old_work
# If new_name contains commas, edit the sed command accordingly
git filter-branch --index-filter 'git ls-files -s | sed "s,\t\"*,&'$new_name'/," | GIT_INDEX_FILE=$GIT_INDEX_FILE.new git update-index --index-info && mv $GIT_INDEX_FILE.new $GIT_INDEX_FILE' HEAD

cd $new_work
git remote add $new_name $old_work
git pull $new_name master
git push