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
