Random thoughts:

  *  Changes to manifest to support:

     +  Trees of wiki pages and tickets
     +  The ability to cap or close a branch

  *  Add the concept of "clusters" to speed the transfer of "tips"
     on a sync.

  *  Auxiliary tables:

     +  tip
     +  phantom
     +  mlink
     +  plink
     +  branch
     +  tree

  * Plink.isprim changed to record:
     +  child is the principal descendent of parent.
     +  child is a branch from parent
     +  child uses parent as a merge

  * tree records
     + type  (code, wiki, ticket)
     + name  (for wiki and ticket only)
     + treeid

  * branch records
     + treeid
     + origin_rid
     + origin_time
     + tip_rid
     + tip_time
     + color

  * website can toggle isprim between principal and branch.
     + How to preserve across rebuild.  A new record type?
     + How to share with other repositories
  * isprim guessed using userid of parent and child.  Change
    in id suggests a branch.  Same id suggests principal.
    For a tie, go with the earliest check-in as the principal