How to merge git repos with history with conventional git commands only

Clone what is to become child projects

git clone git:path/to/repo1
git clone git:path/to/repo2

Move repo contents one level down. By default this should not move hidden files. We don't want to move the .git folder so that is good.

mkdir repo1/repo1
mv repo1/* repo1/repo1/
cd repo1
git add -A
git commit -m "For merging with parent repo"
cd ..

mkdir repo2/repo2
mv repo2/* repo2/repo2/
cd repo2
git add -A
git commit -m "For merging with parent repo"
cd ..

Create parent repo

mkdir parent
cd parent
git init

( Alternatively, clone the parent from wherever you keep it )

git clone git:path/to/parent
cd parent

Pull child projects into parent repo

git pull ../repo1
git pull ../repo2

Done! Now all that is left is to push the parent to the appropriate upstream location.