Specify whether/how GitHub `github:foo/bar` inputs work in fetchTree

[lf-]

A feature to fetch GitHub inputs with builtins.fetchTree "github:foo/bar" was added to Nix 2.19 without an accompanying release note. It is as-yet unclear to me whether this was an intentional change in specified behaviour or a coincidence. However, it is not implemented in 2.18 or Lix.

~ » ns nixpkgs#nixVersions.nix_2_19
warning: unknown experimental feature 'pipe-operator'
warning: unknown setting 'repl-overlays'
~ » nix eval --impure --expr 'builtins.fetchTree "github:lf-/polkadots"'
warning: unknown experimental feature 'pipe-operator'
warning: unknown setting 'repl-overlays'
{ lastModified = 1621318037; lastModifiedDate = "20210518060717"; narHash = "sha256-pb3g/XpCNsDm/IoYqE+TEKENeMHKSPdc9IYH929dpdQ="; outPath = "/nix/store/6r7sib8bbms8yk7bx7yv89a7a8l50vdr-source"; rev = "cbda8e517807b296f772d93e5045fd7d406e6fad"; shortRev = "cbda8e5"; }

~ » ns nixpkgs#nixVersions.nix_2_18
~ » nix eval --impure --expr 'builtins.fetchTree "github:lf-/polkadots"'
warning: unknown experimental feature 'pipe-operator'
warning: unknown setting 'repl-overlays'
error:
       … while calling the 'fetchTree' builtin

         at «string»:1:1:

            1| builtins.fetchTree "github:lf-/polkadots"
             | ^

       … while fetching the input 'file://github:lf-/polkadots'

       error: unable to download 'file://github:lf-/polkadots': URL using bad/illegal format or missing URL (3) URL rejected: Bad file:// URL