Ignore symlinks when formatting

[Lucus16]

Before this, symlinks were replaced with a formatted file rather than formatting the file behind the symlink. The choice was between ignoring symlinks and following them. Following symlinks means potentially formatting files in an unexpected location which seems much worse than needing to wrap the arguments in a readlink -f when symlinks are otherwise ignored.

Fixes #56.

[Lucus16]

Ah, whoops, I broke nixfmt without arguments.

[Lucus16]

That should fix that.

[kirelagin]

One option might be to add -L option to follow symlinks, but, I’d say, that’s too much trouble. Instead, we can just add an example of an invocation that would follow symlinks (find, xargs, readlink, something like this...).

[Lucus16]

I agree with using safe-exceptions and adding an example like nixfmt $(readlink -f $(find -name '*.nix')). Don't have time for it right now though, feel free to fix them and ask for my review.

[tomberek]

We plan to deprecate the directory mode directly in nixfmt and instead encourage use of treefmt.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/formatting-team-meeting-2024-05-28/46126/1