Commit 5d135d555 (prompts: fix pipestatus for jobs prefixed with "not")
introduced a backwards compatibility hack about adding an optional argument
to __fish_print_pipestatus. This hack would break downgrading to fish 3.1.2
if the user copied the new prompt to their config - they would get a backtrace
on every prompt which is arguably worse than the patch's minor improvement.
This does away with the error trace - old fish just won't show the fancy
new pipestatus on `not true`.
Implemented by passing the last $status as the poor man's kwarg, which works
since 3.1.0 (9b86d5dd1 Export all local exported variables in a new scope).
The prompts don't work with fish 3.0.0 or older; downgrading does not seem
too important in general but I think this patch is an okay simplification.
55bc6a27c6