swarm: add "linearizedParent" into commit infos

Previously clients needed to store a graph to linearize and do
complex non-ui work with graph reconstruction and ordering.
So, this commit introduces linearizedParent that gives for each
commit what is the parent message. So clients just needs to
store a list<interactions> and insert new messages after the
announced parent.
This parent is calculated with the equivalent of
`git log --topo-order --date-order`

Change-Id: I10d0809496a9de5b9110fe8164d3d23be1c023d3