[RFC] adding merge-node to parent lines in a commit

Just a random thought presented for flamage...  Would adding the merge
node to each "parent" line in merge nodes be workable?

parent  SHA1-PARENT  SHA1-MERGE-NODE

It's not exactly clear cut I guess, especially in the case of multi-parent
merges, but it would make descending the history slightly easier.   It
would let you look at any merge, and know exactly which commits were
associated with that merge, without needing any repo-id or time
information.  This information is needed at the time of merge anyway, so
recording it rather than leaving it to be calculated later makes some
sense.  Comments?

Cheers,
Sean