org-narrow-to-subtree: prevents showing other parts of file, still shows from other files

1. Have top-level bullets `* a` and `* b`
2. Narrow to subtree in `* a`
3. `helm-org-rifle` for `b`
4. Nothing found, but a `b` in a different file found

I suggest having 3 groupings in that case:
1. In current subtree
2. In current file
3. In other open org-files