Add Comment.safe_in_reply_to to improve comment xml output.

Add Comment.safe_in_reply_to to improve comment xml output.

Now
  be show --xml ID | be-xml-to-mbox | catmutt
shows appropriate linking regardless of missing references or
references to alt-ids in the original comments.  On the other hand,
  be show --xml ID | be import-xml
could alter alt-ids.  If that's a problem we could turn off
save_in_reply_to usage via an option to the xml methods in the future.