use it like that
npx github:/stealify/git-subrepo
dirname $(which git)
#!/bin/bash
npx github:/stealify/git-subrepo "$@"
install it in the same dir as your git command. the << \EoF > is a so called HereDoc in bash google it if you are not aware of that. the slash tells bash to not interpret the variables before writing the file.
install.sh
#!/bin/bash
cat << \EoF > $(dirname $(which git))/git-subrepo
#!/bin/bash
npx github:/stealify/git-subrepo "$@"
EoF
chmod +x $(dirname $(which git))/git-subrepo