commit ceceb93a0b8ab6adbc54879453d12abbbdd9cafe
Author: Jonathan Champ <jrchamp@ncsu.edu>
Date:   Thu Aug 22 17:58:35 2013 -0400

    JRCHAMP Core - Set current-cat-ancestor class on list items similar to the page hierarchy

diff --git a/wp-includes/class-walker-category.php b/wp-includes/class-walker-category.php
index da518c9..8ef3c58 100644
--- a/wp-includes/class-walker-category.php
+++ b/wp-includes/class-walker-category.php
@@ -170,6 +170,13 @@ class Walker_Category extends Walker {
 					} elseif ( $category->term_id == $_current_term->parent ) {
 						$css_classes[] = 'current-cat-parent';
 					}
+					while ( $_current_term->parent ) {
+						if ( $category->term_id == $_current_term->parent ) {
+							$css_classes[] =  'current-cat-ancestor';
+							break;
+						}
+						$_current_term = get_term( $_current_term->parent, $category->taxonomy );
+					}
 				}
 			}