Fix joaotavora#446: don't run sly-editing-mode twice for same .lisp s…

…ource

This could happen if `sly.el` were first autoloaded when visiting that
Lisp source.

Under those conditions the sly-editing-mode would be turned on once
when running `lisp-mode-hook` and once again on autoload due to the
final slime-detecting snippet in sly.el.

* sly.el: (cond ... slime): Don't turn on sly-editing-mode if already
on.

sly.el

@@ -7464,7 +7464,7 @@ and replace `sly-editing-mode' with `slime-lisp-mode-hook'.")))
   (dolist (buffer (buffer-list))
     (with-current-buffer buffer
       (when (eq major-mode 'lisp-mode)
-        (sly-editing-mode 1)
+        (unless sly-editing-mode (sly-editing-mode 1))
         (ignore-errors (and (featurep 'slime) (funcall 'slime-mode -1)))))))
  (t
   (warn