unable to use github pr refs when using git+http

[lolbinarycat]

Describe the bug

the special pull/N/{head,merge} refrences that github creates for each pull request can only be used with github: flakes, not git+http://github.com flakes.

Steps To Reproduce

1. nix build 'git+https://github.com/NixOS/nixpkgs?ref=pull/303399/merge#cargo-shear'

fatal: couldn't find remote ref refs/heads/pull/303399/merge
error:
       … while fetching the input 'git+https://github.com/NixOS/nixpkgs?ref=pull/303399/merge'

       error: program 'git' failed with exit code 128

Expected behavior
the command should succeed.

nix-env --version output
nix-env (Nix) 2.18.1

Additional context

this could cause problems for programs that want to act generically on git repositories, having to special case github to not use git+http adds unnecessary complexity.

Priorities

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[thufschmitt]

You need to use refs/pull/:ID/merge. git does some magic to abstract over branches, tags and arbitrary other references that Nix doesn't replicate.

Closing as a dupe of #3701