fix: show autocmd output when F is in shortmess (#18251)

The default value of including F in 'shortmess' has the unfortunate side
effect of hiding output from autocommands. This is a common source of
confusion and often leads people to think their autocommands are not
working when they are. There is a small snippet in the docs for
'shortmess' indicating that the F flag suppresses autocmd output, but
it's not easy to find if you don't already know to look for it.

This commit removes that behavior of the F flag to make it only suppress
file info when opening a new file.

src/nvim/ex_cmds.c

@@ -1387,7 +1387,7 @@ static void do_filter(linenr_T line1, linenr_T line2, exarg_T *eap, char_u *cmd,
   if (do_out) {
     if (otmp != NULL) {
       if (readfile(otmp, NULL, line2, (linenr_T)0, (linenr_T)MAXLNUM, eap,
-                   READ_FILTER) != OK) {
+                   READ_FILTER, false) != OK) {
         if (!aborting()) {
           msg_putchar('\n');
           semsg(_(e_notread), otmp);