much simpler .gitignore for ext/

much simpler .gitignore for ext/

.gitignores are advisory, they affect the "Untracked files" list and the behaviour of git add $path,
and nothing else. A tracked file is tracked, and explicitly adding a file overrules any .gitignore file,
so we can make the ext/.gitignore quite restrictive, and assume that any changes will be done by something
that will explicitly add the required files.