The new builder makes it possible to break cycles that occurs when the
fixed-output derivation for the source of a dependency of ‘git’ would
itself depend on ‘git’.
* guix/scripts/perform-download.scm (perform-git-download): New
procedure.
(perform-download): Move fixed-output derivation check to…
(guix-perform-download): … here. Invoke ‘perform-download’ or
‘perform-git-download’ depending on what ‘derivation-builder’ returns.
* nix/libstore/builtins.cc (builtins): Add “git-download”.
* tests/derivations.scm ("built-in-builders"): Update.
("'git-download' built-in builder")
("'git-download' built-in builder, invalid hash")
("'git-download' built-in builder, invalid commit")
("'git-download' built-in builder, not found"): New tests.
