Migration strategy for changing `github:` inputs to `git:`-backed implementation

[roberth]

Is your feature request related to a problem?

Shallow git-based fetching appears to have better performance than tarball-based fetching.

They're not exactly the same, so we'll have to manage this carefully.

Proposed solution

Make use of original vs locked fields in the lock file.

Add a flag to control what's generated during locking.

New setting
Default: current behavior
Opt in: generate git-based lock node

This should be a setting, because then we can test it before we start switching the ecosystem in a future release.

Alternative solutions

Additional context

• Some more discussion in Tar download very slow #10683

(Nix 2.24 or 2.25 did improve tarball cache performance, but it is still outperformed by git, it seems)

Checklist

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

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[nixos-discourse]

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

https://discourse.nixos.org/t/2025-05-07-nix-team-meeting-minutes-224/63973/1