lib.fileset.trace should show whether directories include files by default

[infinisil]

Issue description

lib.fileset.trace should show whether directories include files by default.

Here's an example:

$ mkdir d d/d
$ touch f d/f d/d/f

$ nix repl -f channel:nixos-unstable
[...]

nix-repl> :a lib.fileset                                
Added 11 variables.

nix-repl> trace (difference ./. (difference ./d ./d/d))
trace: /home/tweagysil/test/x
trace: - d
trace:   - d (all files in directory)
trace: - f (regular)

The trace should instead be something like this:

trace: /home/tweagysil/test/x (includes files by default)
trace: - d (excludes files by default)
trace:   - d (all files in directory, includes files by default)
trace: - f (regular)

This is totally doable, just requires a bunch of time and might make everything slower (not just the tracing). So I'm not sure if this is worth having. For now just writing this down to track it.

This issue is sponsored by Antithesis ✨