Make it possible to clear a PlotCommitList

This allows SwingGraphPanes to be reused by simply clearing and
re-filling. Requires RevObjectList initialization to not call clear()
from its constructor, because this will lead PlotCommitList.clear()
to be called before all variables have been initialized.

Change-Id: I14a07124441b58cd88c67da088ba52ef9c30b043
Signed-off-by: Jonas Fonseca <fonseca@diku.dk>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>