Re: the opposite of .gitignore, whitelist

One other option is to just use a pattern that matches everything, i.e:
    echo '*' > .gitignore

And take advantage that ignore rules do not apply to tracked files.
So instead of using an explicit .gitonly, you add files using:
   git add -f <file_to_be_included>

All files should be ignored except the ones that were forcibly added.

If needed, git ls-files can be used to list either category.

Cheers, Rafael Ascensão