obshistory: implement predecessorsandmarkers() for obsorigin

fold: proposal for output improvement Listing other changeset we got folded with seems like a good idea. We need to think about the case where changesets we got folded with is filtered.

obslog: add display of precursor information to the default template This will still only be output when the user passes --precursors to the command (otherwise the data just won't be there). This addresses a common user request, where they actually would find more intuitive to use --precursors --no-successors.