racket-repl-mode: Default font-lock-keywords-only to true; fixes #751

This means that font-lock will no longer highlight strings or comments
in REPL buffers when used by classic racket-mode. The advantage: It
won't MIS-highlight them due to stray string or comment delimiters
among the various kinds of REPL output.

Author: greghendershott

racket-hash-lang.el

@@ -853,6 +853,7 @@ rhombus\"."
       ;; font-lock
       (setq-local font-lock-keywords
                   (with-current-buffer edit-buffer font-lock-keywords))
+      (setq-local font-lock-keywords-only t) ;#751
       (setq-local racket--repl-fontify-region-function
                   (with-current-buffer edit-buffer font-lock-fontify-region-function))
       (font-lock-flush)