add markdown-command-needs-filename to async

Author: Danny McClanahan

markdown-mode.el

@@ -6025,9 +6025,14 @@ alive."
             (proc
              (start-process-shell-command
               markdown-live-preview-proc-name markdown-live-preview-buf-name
-              ;; using redirection means emacs doesn't have to process output
+              ;; using redirection means emacs doesn't have to process output,
+              ;; which interrupts the user less
               (concat
-               markdown-command " > " (shell-quote-argument out-file)))))
+               markdown-command
+               (and markdown-command-needs-filename
+                    (if (buffer-file-name) (buffer-file-name)
+                      (user-error "Must be visiting a file")))
+               " > " (shell-quote-argument out-file)))))
        (setq markdown-live-preview-currently-exporting-process proc)
        (set-process-sentinel proc sentinel)
        (with-temp-buffer