Fix PlotCommitList to set lanes on child-less commits

Fix PlotCommitList to set lanes on child-less commits

In PlotCommitList.enter() commits are positioned on lanes for visual
presentation. This implementation was buggy: commits without
children (often the starting points for the RevWalk) are not positioned
on separate lanes.

The problem was that when handling commits with multiple children
(that's where branches fork out) it was not handled that some of the
children may not have been positioned on a lane yet. I fixed that and
added a number of tests which specifically test the layout of commits
on lanes.

Bug: 300282
Bug: 320263
Change-Id: I267b97ecccb5251cec54cec90207e075ab50503e
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>