substitute: Make download messages more concise.

'guix substitute' now displays 2 lines per download instead of 4.
Suggested by Ricardo Wurmus.

* guix/scripts/substitute.scm (assert-valid-narinfo): Have #:verbose?
default to #f.  Remove leading newline in message.
(process-substitution): Display the URI rather than the store file name.
Display two newlines after the substitution.

guix/scripts/substitute.scm

@@ -391,7 +391,7 @@ (define (narinfo-sha256 narinfo)
 
 (define* (assert-valid-narinfo narinfo
                                #:optional (acl (current-acl))
-                               #:key (verbose? #t))
+                               #:key verbose?)
   "Raise an exception if NARINFO lacks a signature, has an invalid signature,
 or is signed by an unauthorized key."
   (let ((hash (narinfo-sha256 narinfo)))
@@ -404,9 +404,8 @@ (define* (assert-valid-narinfo narinfo
           (unless %allow-unauthenticated-substitutes?
             (assert-valid-signature narinfo signature hash acl)
             (when verbose?
-              ;; Visually separate substitutions with a newline.
               (format (current-error-port)
-                      (_ "~%Found valid signature for ~a~%")
+                      (_ "Found valid signature for ~a~%")
                       (narinfo-path narinfo))
               (format (current-error-port)
                       (_ "From ~a~%")
@@ -893,7 +892,7 @@ (define* (process-substitution store-item destination
             ;; "(4.1MiB installed)"; it shows the size of the package once
             ;; installed.
             (_ "Downloading ~a~:[~*~; (~a installed)~]...~%")
-            (store-path-abbreviation store-item)
+            (uri->string uri)
             ;; Use the Nar size as an estimate of the installed size.
             (narinfo-size narinfo)
             (and=> (narinfo-size narinfo)
@@ -921,8 +920,9 @@ (define* (process-substitution store-item destination
       ;; Unpack the Nar at INPUT into DESTINATION.
       (restore-file input destination)
 
-      ;; Skip a line after what 'progress-proc' printed.
-      (newline (current-error-port))
+      ;; Skip a line after what 'progress-proc' printed, and another one to
+      ;; visually separate substitutions.
+      (display "\n\n" (current-error-port))
 
       (every (compose zero? cdr waitpid) pids))))