If you want to merge project-a into project-b:

cd path/to/project-b
git remote add project-a path/to/project-a
git fetch project-a --tags
git merge --allow-unrelated-histories project-a/master # or whichever branch you want to merge
git remote remove project-a

Taken from: git merge different repositories?

This method worked pretty well for me, it's shorter and in my opinion a lot cleaner.

In case you want to put project-a into a subdirectory, you can use git-filter-repo (filter-branch is discouraged). Run the following commands before the commands above:

cd path/to/project-a
git filter-repo --to-subdirectory-filter project-a

An example of merging 2 big repositories, putting one of them into a subdirectory: https://gist.github.com/x-yuri/9890ab1079cf4357d6f269d073fd9731

Note: The --allow-unrelated-histories parameter only exists since git >= 2.9. See Git - git merge Documentation / --allow-unrelated-histories

Update : Added --tags as suggested by @jstadler in order to keep tags.