Merge repos and keep whole history by creating a single multi-parent merge commit

git_merge_repos.sh

#!/bin/bash

set -e
set -u
set -o pipefail

prefix="git@gitlab.somewhere.com:mygroup"

target="$1"
shift
repos="$*"

if [[ -z $repos ]]; then
    echo "usage: $0 repo [repo ...]" 1>&2
    exit 1
fi

# clone locally as cache
for repo in $repos; do
    if [[ ! -d cache/$repo ]]; then
        git clone "$prefix/$repo.git"  "cache/$repo"
    fi
done

rm -rf "$target"

# make a repo with each source as remote
git init "$target"
cd "$target"
for repo in $repos; do
    git remote add "$repo"  "$PWD/../cache/$repo"
done
for repo in $repos; do
    git fetch --no-tags "$repo"
done

# checkout each one in a subdir and make a commit on a dedicated branch materialising the move to subdir on each
for repo in $repos; do
    mkdir -p "$repo"
    git --work-tree="$repo" checkout "$repo"/master
    git checkout -b "$repo"
    git add .
    git commit -m "Move $repo into subdir"
    git checkout --orphan master
    git reset --hard
    rm -rf "$repo"
done

p_args="$(echo "$repos" | perl -ne 's/ /\n/g and print' | perl -ne '/(.*)/ and { print "-p " and system("git rev-parse $1") }' | xargs echo)"

# checkout all of them in a subdir
for repo in $repos; do
    mkdir -p "$repo"
    git --work-tree="$repo" checkout "$repo"/master
done

# make a merge commit of all dedicated move branches
git checkout --orphan master
git reset
git add .
tree=$(git write-tree)
echo "$p_args"
commit=$(git commit-tree "$tree" $p_args -m "Merge of $(echo "$repos" | wc -w | sed 's/ //g') repos $(echo -e "\n"; echo "$repos" | perl -ne 's/ /\n/g and print')")
git reset --hard "$commit"

# finalize
git remote add origin "$prefix/$target"
git fetch --no-tags origin
git branch -u origin/master