Ensure the tree parser resets in ObjectWalk

During dispose() or reset() we are suppose to be restoring the
ObjectWalk instance back to the original pre-walk state, but we
failed to reset the tree parser.  This can lead to confusing state
if the ObjectWalk was reused by the caller, as entries from the
old walk might be reported as part of the new walk.

Change-Id: I6237bae7bfd3794e8b9a92b4dd475559cc72e634
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>