lua: add {old_byte_size} to on_lines buffer change event

src/nvim/buffer_defs.h

@@ -807,6 +807,9 @@ struct file_buffer {
   kvec_t(uint64_t) update_channels;
   kvec_t(BufUpdateCallbacks) update_callbacks;
 
+  size_t deleted_bytes;
+  int flush_count;
+
   int b_diff_failed;    // internal diff failed for this buffer
 };