merge-unrelated.md

git merge --allow-unrelated-histories

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

Source: StackOverflow answer #10548919.

I merged unrelated gist histories for this repository, and Git history timeline looks like this: