Avoid expensive getAllRefsByPeeledObjectId() in PlotWalk constructor

Avoid expensive getAllRefsByPeeledObjectId() in PlotWalk constructor

Instead, do it when we return the first PlotCommit from next().
On a repository with many refs, getAllRefsByPeeledObjectId() can
take a while. Doing a late initialization simplifies the handling
of a PlotWalk.

EGit, for instance, creates and configures an instance, and then
does the real walk in a background job. With late initialization,
the potentially expensive getAllRefsByPeeledObjectId() also occurs
in that background job.

Bug: 485743
Change-Id: I84c020cf8f7afda6f181778786612b8e6ddd7ed8
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>