bug#47271: guix graph --path results in backtrace

[Mark H Weaver <mhw@netris.org>]

This is at commit 1955ef93b76e51cab5bed4c90f7eb9df7035355a on the master
branch, plus some local patches on my private branch which I suspect are
irrelevant to this:

--8<---------------cut here---------------start------------->8---
mhw@jojen ~$ guix graph --path gtk+ imagemagick
Backtrace:
In ice-9/boot-9.scm:
  1736:10 13 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
In unknown file:
          12 (apply-smob/0 #<thunk 7fc164214520>)
In ice-9/boot-9.scm:
    718:2 11 (call-with-prompt _ _ #<procedure default-prompt-handler (k proc)>)
In ice-9/eval.scm:
    619:8 10 (_ #(#(#<directory (guile-user) 7fc164217c80>)))
In guix/ui.scm:
  2164:12  9 (run-guix-command _ . _)
In guix/scripts/graph.scm:
    573:2  8 (guix-graph . _)
In ice-9/boot-9.scm:
  1736:10  7 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
  1731:15  6 (with-exception-handler #<procedure 7fc1619c88d0 at ice-9/boot-9.scm:1815:7 (exn)> _ #:unwind? _ #:unwind-for-type _)
  1736:10  5 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
In guix/store.scm:
   636:37  4 (thunk)
In guix/scripts/graph.scm:
   598:37  3 (_ #<store-connection 256.99 7fc1619d4780>)
   598:37  2 (_ #f)
In ice-9/boot-9.scm:
  1669:16  1 (raise-exception _ #:continuable? _)
  1669:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1669:16: In procedure raise-exception:
Wrong type to apply: #<syntax-transformer set-grafting>
mhw@jojen ~$
--8<---------------cut here---------------end--------------->8---