Merge 3 git repositories into one having the same (empty) initial commit

merge-sim-project.sh

#!/bin/bash
set -exuo pipefail

rm -rf sim-project
git init sim-project
cd sim-project

git commit -m 'Initial commit' --allow-empty
INITIAL_COMMIT=$(git rev-parse @)

# sim
git clone https://github.com/varqox/sim sim-git
(cd sim-git && git filter-repo --to-subdirectory-filter sim)
git remote add sim sim-git/
git fetch sim main develop --no-tags
INITIAL_SIM_COMMIT=$(git rev-list --first-parent sim/main | tail -1)
git replace $INITIAL_SIM_COMMIT --graft $INITIAL_COMMIT

# simlib
git clone https://github.com/varqox/simlib simlib-git
(cd simlib-git && git filter-repo --to-subdirectory-filter simlib)
git remote add simlib simlib-git/
git fetch simlib main --no-tags
INITIAL_SIMLIB_COMMIT=$(git rev-list --first-parent simlib/main | tail -1)
git replace $INITIAL_SIMLIB_COMMIT --graft $INITIAL_COMMIT

# sip
git clone https://github.com/varqox/sip sip-git
(cd sip-git && git filter-repo --to-subdirectory-filter sip)
git remote add sip sip-git/
git fetch sip main --no-tags
INITIAL_SIP_COMMIT=$(git rev-list --first-parent sip/main | tail -1)
git replace $INITIAL_SIP_COMMIT --graft $INITIAL_COMMIT

# Rewrite history to "rebase" all branches onto the $INITIAL_COMMIT
git filter-repo --path-rename ":" --force

git merge sim/main
git merge simlib/main
git merge sip/main

git checkout sim/develop
git rebase main
git branch develop
git checkout main

git remote remove sim
git remote remove simlib
git remote remove sip
rm -rf sim-git/
rm -rf simlib-git/
rm -rf sip-git/