scalafmt: Add --stdout as workaround (#238)

Sometimes, scalafmt does not produce formatted output to stdout with
only the `--stdin` flag provided. For example, this happens when there
are no formatting changes required to be made to the file. As-is, this
causes the formatter to clear the buffer when there are no formatting
changes required.

Fix by adding the `--stdout` flag.

See scalameta/scalafmt#3632 for more
information.

format-all.el

@@ -1266,7 +1266,7 @@ Consult the existing formatters for examples of BODY."
   (:features)
   (:format
    (format-all--buffer-easy
-    executable "--stdin" "--non-interactive" "--quiet")))
+    executable "--stdin" "--non-interactive" "--quiet" "--stdout")))
 
 (define-format-all-formatter shfmt
   (:executable "shfmt")