run the `__complete` probe subprocess in its own session so that a
cobra-incompatible binary that ignores `__complete` can't mess with the
user's controlling terminal.
tested on both macos and nixos.
encountered the problem while trying to use
[visidata](https://www.visidata.org/).
---------
Co-authored-by: shemishtamesh <shemishtamail@gmail.com>
