Isolate a single folder from an existing repository and add it to another repo, keeping history.txt

Make a fresh copy of both the repo to strip down, and the repo you wish to merge into. 

In the repo you want to strip down: 

git filter-branch --prune-empty --subdirectory-filter <subdir of folder you want to keep> <branch to isolate from> 

You will now have a stripped-down copy of your repo, with all history. You may need to clean up adjacent folders/files. 

Create a new branch and commit any changes you have made. 

If this was your only goal, celebrate!

If you wanted to merge this folder into a different, existing repo: 

cd <path of repo to merge INTO> 

Create a new branch in your repo to merge into for your merging

git remote add <arbitrary name for your new folder project> <path to new folder project> 
git fetch <arbitrary name> --tags 
git merge --allow-unrelated-histories <arbitrary name>/<branch to merge> 
git remote remove <arbitrary name> 


Resources: 
https://stackoverflow.com/a/10548919/5806755
https://help.github.com/en/articles/splitting-a-subfolder-out-into-a-new-repository