merge_git_repo_as_subdir.sh

# based on the following:
# http://saintgimp.org/2013/01/22/merging-two-git-repositories-into-one-repository-without-losing-file-history/
# http://blog.caplin.com/2013/09/18/merging-two-git-repositories/

git clone repo_main
git clone repo_sub
cd repo_main

git remote add repo_sub ../repo_sub
git fetch repo_sub

git checkout -b repo_sub repo_sub/master
mkdir dir_repo_sub

# '*' will cause git mv to fail because it cannot move dir_repo_sub into itself
git mv <list all files>
git commit -m"Moves all repo_sub files into a single directory for merging back into repo_main"

git checkout master
git merge repo_sub

# commit and push, and you're finished

in case of this error fatal: refusing to merge unrelated histories, check the below link:
https://stackoverflow.com/questions/37937984/git-refusing-to-merge-unrelated-histories-on-rebase