fetchgit: fetch submodules in parallel

# Update submodules

init_submodules(){

    # Add urls into .git/config file

    clean_git submodule init

    # list submodule directories and their hashes

    git submodule status |

    while read -r l; do

        local hash

        local dir

        local name

        local url

        # checkout each submodule

        hash=$(echo "$l" | awk '{print $1}' | tr -d '-')

        dir=$(echo "$l" | sed -n 's/^.[0-9a-f]\+ \(.*[^)]*\)\( (.*)\)\?$/\1/p')

        name=$(

            git config -f .gitmodules --get-regexp submodule\..*\.path |

            sed -n "s,^\(.*\)\.path $dir\$,\\1,p")

        url=$(git config --get "${name}.url")

        clone "$dir" "$url" "$hash" ""

    done

    clean_git submodule update --init --recursive -j $NIX_BUILD_CORES

}

clone(){