fix(diff): "nvim -d" should only diff arglist files #21829

Co-authored-by: 0xadk <0xadk@users.noreply.github.com>

src/nvim/main.c

@@ -539,9 +539,11 @@ int main(int argc, char **argv)
   if (params.diff_mode) {
     // set options in each window for "nvim -d".
     FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
+      if (!wp->w_arg_idx_invalid) {
         diff_win_options(wp, true);
       }
     }
+  }
 
   // Shorten any of the filenames, but only when absolute.
   shorten_fnames(false);