Move WorkingTreeIterator inherited state into an object

Move WorkingTreeIterator inherited state into an object

Instead of copying up to 4 fields from the parent iterator each time a
child iterator is initialized and used, construct a single state
object that contains the 4 fields, and pass that one state object
through to the child.  This makes it easier to add additional state
fields that must be inherited, at the slight expense of an extra
object allocation per TreeWalk, and an extra level of field
indirection whenever the options, nameEncoder, or read buffer is
required by the iterator.

Change-Id: Ic4603c33b772d7a45f9c81140537d51945688fcb
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>