I want to add an external directory to an existing repository.
External Dir: /home/some/directory
Working Dir: /htdocs/.git
If I attempt the following command from the /htdocs dir:
git add /home/some/directory
I get an error: fatal: '/home/some/directory' is outside repository
If I need to do something like that I would normally move that external file or directory into my git repo and symlink it's original location to the new one.
mv /home/some/directory /htdocs/directory
ln -s /htdocs/directory /home/some/
git add ./directory
I use this technique when I am developing a plug-in for an application that I want to keep under version control but have to store in a specific location.