Relativize paths only if subpath

Editor path should only become relative when saving if its a subpath of the parent dir of the file

Otherwise when you select very arbitrary file paths chaos ensues with things like `../../../../../`

Same goes for all relativized paths

Start at the editor's save-path handling and locate the existing logic that relativizes paths. Reproduce the case where an arbitrary file path is selected, then verify that only paths under the parent directory become relative and other paths remain unchanged, including the equivalent behavior for other relativized paths.

Written by the indexing model from the issue text.