This allows for using a package source directly from git, doing something like

    (define (command->bytevector command)
      (let ((port (apply open-pipe* OPEN_READ command)))
        (let ((output (get-bytevector-all port)))
          (close-port port)
          output)))

    (define-public hello-git
      (package
        (name "hello")
        (version "git")
        (source (let* ((commit "stable-2.0")
                       (content (command->bytevector
                                 `("git" "archive" "--format" "tar" "--prefix"
                                   ,(string-append commit "/") ,commit)))
                       (file-name (string-append "hello-" commit)))
                  (plain-file file-name content)))
        ...
        ))

* guix/gexp.scm (<plain-file>): Also allow bytevector content.
(plain-file-compiler): Handle bytevector content.
* doc/guix.texi (G-Expressions): Describe plain-file now also taking
bytevectors.