Track LastBlock correctly in traversal.Focus.

And test.
Signed-off-by: Eric Myhre <hash@exultant.us>

traversal/focus.go

@@ -78,6 +78,8 @@ func (tp TraversalProgress) Focus(n ipld.Node, p ipld.Path, fn VisitFn) error {
 			if err != nil {
 				return fmt.Errorf("error traversing node at %q: could not load link %q: %s", p.Truncate(i+1), lnk, err)
 			}
+			tp.LastBlock.Path = p.Truncate(i + 1)
+			tp.LastBlock.Link = lnk
 			prev, n = n, next
 		}
 	}

traversal/focus_test.go

@@ -150,6 +150,8 @@ func TestFocusWithLinkLoading(t *testing.T) {
 		}.Focus(rootNode, ipld.ParsePath("linkedMap/nested/nonlink"), func(tp traversal.TraversalProgress, n ipld.Node) error {
 			Wish(t, n, ShouldEqual, fnb.CreateString("zoo"))
 			Wish(t, tp.Path, ShouldEqual, ipld.ParsePath("linkedMap/nested/nonlink"))
+			Wish(t, tp.LastBlock.Link, ShouldEqual, middleMapNodeLnk)
+			Wish(t, tp.LastBlock.Path, ShouldEqual, ipld.ParsePath("linkedMap"))
 			return nil
 		})
 		Wish(t, err, ShouldEqual, nil)