vim-patch:9.1.0231: Filetype may be undetected when SwapExists sets ft in other buf (#28136)

Problem:  Filetype may be undetected when a SwapExists autocommand sets
          filetype in another buffer.
Solution: Make filetype detection state buffer-specific.  Also fix a
          similar problem for 'modified' (zeertzjq).

closes: vim/vim#14344

5bf6c2117f

src/nvim/option.c

@@ -2244,7 +2244,7 @@ static const char *did_set_modified(optset_T *args)
     save_file_ff(buf);  // Buffer is unchanged
   }
   redraw_titles();
-  modified_was_set = (int)args->os_newval.boolean;
+  buf->b_modified_was_set = (int)args->os_newval.boolean;
   return NULL;
 }