Reference:
https://github.com/ziglang/zig/pull/19500#discussion_r1556476973

Arena is now used for Diagnostic (tar and git). `deinit` is not called on Diagnostic
allowing us to reference strings from Diagnostic in UnpackResult without
dupe.

That seamed reasonable to me. Instead of using gpa for Diagnostic, and
then dupe to arena. Or using arena for both and making dupe so we can deinit
Diagnostic.