Pull gitlibs like clj cli does it. Helps to pull in git deps in a sort of immutable spirit, if you have another ref, it pulls the other ref next to the existing one.

procure-git.sh

#!/bin/sh

if [ -z "$1" ] || [ -z "$2" ]; then
    echo "Usage: $0 <git-repo> <ref>"
    exit 1
fi

repo=$(echo $1 | sed -e 's/.*\///' -e 's/\.git$//')

mkdir -p .gitlibs/
if [ ! -d .gitlibs/$repo ]; then
    git clone --quiet --mirror $1 .gitlibs/$repo
fi

if [ ! -d .gitlibs/$repo/$2 ]; then
    git --git-dir .gitlibs/$repo worktree add --force --detach .gitlibs/$repo/$2 $2
fi
echo .gitlibs/$repo/$2