profiles: Use a real arrow to denote upgrades in ASCII.

Suggested by Alex Kost.

* guix/profiles.scm (right-arrow): Fall back to "->".
* tests/profiles.scm ("manifest-show-transaction"): Adjust accordingly.

guix/profiles.scm

@@ -326,7 +326,7 @@ replacement if PORT is not Unicode-capable."
               (lambda ()
                 (display arrow)))))
         (lambda (key . args)
-          ">")))))
+          "->")))))
 
 (define* (manifest-show-transaction store manifest transaction
                                     #:key dry-run?)

tests/profiles.scm

@@ -167,7 +167,7 @@
                              (with-error-to-string
                               (lambda ()
                                 (manifest-show-transaction store m t)))))
-             (string-match "guile\t1.8.8 > 2.0.9"
+             (string-match "guile\t1.8.8 -> 2.0.9"
                            (with-fluids ((%default-port-encoding "ISO-8859-1"))
                              (with-error-to-string
                               (lambda ()