git-import-subtree

#!/bin/bash
source "$(git --exec-path)/git-sh-setup"

USAGE=$(cat <<EOF
<destination> <remote> <ref>
    Create a new directory named <destination> containing the contents of the
    specified <remote> repository at the specified <ref>, preserving the
    history of the remote repository.
EOF
)

function __subtree_import() {
    if [[ $# == 3 ]]; then
        local destination=$1
        local remote=$2
        local ref=$3

        git remote add -f $destination $remote
        git merge -s ours --no-commit --allow-unrelated-histories $ref
        git read-tree --prefix=$destination -u $ref
        git commit -am "$destination: import from $remote $ref"
    else
        usage
    fi
}

__subtree_import $@