Move one repository into another (merging) without losing history

gistfile1.txt

# Caution:   new repo should be empty otherwise you lose new repo data!


git clone --mirror <url_of_old_repo>
cd <name_of_old_repo.git>
git remote add new-origin <url_of_new_repo>
git push new-origin --mirror


# if you want merge two repositories you should use the following commands
# merging project-a into the project-b

cd <path_to_project_b>
git remote add project-a <url_of_project_a>
git fetch project-a
git merge --allow-unrelated-histories project-a/master # or whichever branch you want to merge
git push origin master