fix data overlay definitions.

when using overlay files to modify base definitions for client or site specific data, the base table rows are not being indexed properly and causing duplicate <rows> <row> elements to be added when primary key overlaying should be occuring.

updated IdenticalDiffSameOutputTest to test overlaying of files

actual fix diff is
@@ -521,7 +528,7 @@ class xml_parser {
     // primary key key => row index
     $base_pklookup = array();
     $i = 0;
-    foreach ($base_table->row as $base_row) {
+    foreach ($base_table_rows->row as $base_row) {
       $s = '';
       foreach ($primary_key_index['base'] as $index) {
         $s .= ':'.$base_row->col[$index];