Move ignore node handling into WorkingTreeIterator

Move ignore node handling into WorkingTreeIterator

The working tree iterator has perfect knowledge of the path structure
as well as immediate information about whether or not an ignore file
even exists at this level.  We can exploit that to simplify the
logic and running time for testing ignored file status by pushing
all of the checks down into the iterator itself.

Change-Id: I22ff534853e8c5672cc5c2d9444aeb14e294070e
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
CC: Charley Wang <chwang@redhat.com>
CC: Chris Aniszczyk <caniszczyk@gmail.com>
CC: Stefan Lay <stefan.lay@sap.com>
CC: Matthias Sohn <matthias.sohn@sap.com>