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

[Jonta]

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