Merge unrelated git repositories into monolith and preserve history

gistfile1.txt

Goal:

Move vendor repository to "src" directory of the project while keeping git files history.

Folder: buzz-platform
git checkout -b release-pop

Navigate outside the repository:
cd ../

Clone first vendor repo
git clone [email protected]:trivago/buzz-frontend-core-bundle.git frontend

Remove all files except the desired sub folder
cd frontend
git filter-branch --subdirectory-filter src/Buzz -- --all

Return to buzz-platform and create a origin pointing to new clean FrontendCoreBundle that is going to be merged 
cd ../buzz-platform
git remote add frontend ../frontend
git fetch frontend
git merge --allow-unrelated-histories frontend/release
git mv FrontendCoreBundle src/Buzz/