This simple script allows to split a folder from a project (make a backup first!), and will keep all branches and valid tags that use this folder.

#!/bin/bash

URL=$1
FOLDER=$2
SPLITTER=$3

set -e

function git_extract(){
     pushd $1

     git remote rm origin

     echo "Extracting $2 from $1"
     echo "Original size $(du -h -s .)"
     for TAG in $( git tag -l ); do
         if [ -z "$(git ls-tree refs/tags/$TAG $2)" ]; then
            echo "$TAG will be deleted"
            git tag -d $TAG
         fi;
     done

     for BRANCH in $( git branch -a | grep -v master | grep -v branches ); do
         if [ -z "$(git ls-tree refs/heads/$BRANCH $2)" ]; then
            echo "$BRANCH will be deleted"
            git branch -D $BRANCH
         fi;
     done

     git filter-branch \
           --subdirectory-filter $2 \
           --tag-name-filter cat \
           --prune-empty -- --all
     rm -rf refs/original

     if [ "$3" == "mark-split" ]; then
         git filter-branch --msg-filter '
         cat &&;
         echo ""
         echo "Splitted-from: $4"
         echo "Splitted-by: $5"
        ' --tag-name-filter cat -- --all
     fi
     rm -rf refs/original logs lost-found
     git reflog expire --all
     git gc --prune=0 --aggressive # shrink the project please!
     echo "new size $(du -h -s .)"
     popd
}

git clone --mirror --bare $URL split-base-mirror

git_extract split-base-mirror $FOLDER mark-split $URL $SPLITTER

The script takes 3 arguments: url from which we get the repo, folder we’re splitting and your Name for tagging it.