Fix a use of `fresh-line'.

Needs to do it when the relevant stderr is in effect.

Fixes PR 13744, except that something like this is still bad:

  (with-output-to-file "/tmp/foo" #:exists 'append
    (lambda ()
      (parameterize ([current-error-port (current-output-port)])
        (/ 1 0))))

But I think that it's a bad idea for the error display handler to be
called with the locally set port -- described in PR 13974.

pkgs/xrepl-pkgs/xrepl-lib/xrepl/xrepl.rkt

@@ -1457,7 +1457,6 @@
 ;; a display handler that omits stacktraces (making them available later)
 
 (define ((make-xrepl-display-handler orig) str exn)
-  (fresh-line #t)
   (define backtrace?
     (parameterize ([current-error-port (open-output-string)])
       (orig str exn)
@@ -1477,6 +1476,7 @@
            [s (if backtrace?
                 (string-append s (if (regexp-match? #rx"\n" s) "\n; " " ") msg)
                 s)])
+      (fresh-line #t)
       (with-wrapped-output (printf "; ~a\n" s)))))
 
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