merge_repos.sh

#!/bin/bash
set -euo pipefail

FROM=$1
TO=$2
NEW_NAME=${3:-$FROM}

cd "$FROM"
FROM=$(pwd)
# Uses https://github.com/newren/git-filter-repo
git filter-repo --to-subdirectory-filter "$NEW_NAME/"

cd "../$TO"
git remote add "$NEW_NAME" "$FROM"
git fetch "$NEW_NAME"

git checkout master
git merge --allow-unrelated-histories "$NEW_NAME"/main

git remote remove "$NEW_NAME"