Detach subdirectory from Git repository as separate repo

git-detach.sh

#!/bin/sh

# Usage: git-detach <directory> <target>
# Creates a new Git repository at <target> from the contents of <directory>
# and its history. Does not remove the directory from its original repo.
#
# E.g.: suppose project/ is a Git repository with a subdirectory lib/, then
#   git-detach project/lib/ standalone-lib/
# creates a new repository standalone-lib/ holding the contents of project/lib/
# as a separate repo.

if [ $# != "2" ]; then
    echo "Usage: git-detach <directory> <target>" >&2
    exit 1
fi

set -e #-x

src="$1"
tgt="$2"

subdir=$(cd "$src" && git rev-parse --show-prefix)

git clone $(cd "$src" && git rev-parse --show-toplevel) "$tgt"
cd "$tgt"
git filter-branch --subdirectory-filter "$subdir" --prune-empty

I was previously doing this stuff manually. Now I accidentally stumbled upon your gist.
It worked flawlessly! Brilliant!
Thank you!