fetchTree sets shallow = true

[edolstra]

Describe the bug

fetchTree since 358c26f has this ad hoc treatment of the shallow attribute:

        // fetchTree should fetch git repos with shallow = true by default
        if (type == "git" && !params.isFetchGit && !attrs.contains("shallow")) {
            attrs.emplace("shallow", Explicit<bool>{true});
        }

This is bad because it makes fetchTree behave inconsistently with Git repos fetched in other ways. For example, when you add a flake input like

  inputs.nixpkgs = { type = "git"; url = "git@github.com:NixOS/nixpkgs.git"; };

running a build/eval on this flake the first time will result in a cache path like

/home/eelco/.cache/nix/gitv3/1is1wr7j5dq4hnf6yhzrj2d6zv0874crgjrpg5jd9nwnkxzyg1fw

but the second time you get

/home/eelco/.cache/nix/gitv3/1is1wr7j5dq4hnf6yhzrj2d6zv0874crgjrpg5jd9nwnkxzyg1fw-shallow

(i.e. the repo will be cloned again.) This is because fetchTree is only used internally when an input is already locked. Similarly, nix flake metadata git+ssh://git@github.com/NixOS/nixpkgs.git doesn't use fetchTree.

builtins.fetchTree should never mess with the input attributes like this. Doing a shallow clone should be an optimization done in the git fetcher, if appropriate (e.g. if the revCount is already specified).

Steps To Reproduce

Expected behavior

Metadata

Additional context

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

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