Allow AbstractTreeIterator subclasses to supply their own path array

Allow AbstractTreeIterator subclasses to supply their own path array

Some forms of AbstractTreeIterator may already have the path buffer
they want their child iterator to use in memory when the child is
being created.  Therefore they do not want the existing constructor
as it would modify the parent's path buffer, and force the subclass
to copy the new child's path.

By allowing subclass implementations to define their own path buffer
we must rely on them to do the right thing in terms of setting up a
valid buffer for iteration, but the implementation can avoid copies
and unwanted data changes to the parent array.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>