Bare with the following, as I tried to think of a simple example and a common web structure came to mind. If the structure is like this:
/www
|-- scripts
|-- js
`shared_code.js
`-- css
`shared_style.css
|-- images
`-- cgi-bin/
|-- projectA
|-- .git
|-- foo.php
`-- foo.pl
`-- projectB
|-- .git
|-- bar.php
`-- bar.pl
Is it possible to add the js/css scripts to the git repositories?