Add support for recursive graph bisection.

lucene/core/src/java/org/apache/lucene/index/SortingCodecReader.java

@@ -333,7 +333,7 @@ public static CodecReader wrap(CodecReader reader, Sort sort) throws IOException
    * Expert: same as {@link #wrap(org.apache.lucene.index.CodecReader, Sort)} but operates directly
    * on a {@link Sorter.DocMap}.
    */
-  static CodecReader wrap(CodecReader reader, Sorter.DocMap docMap, Sort sort) {
+  public static CodecReader wrap(CodecReader reader, Sorter.DocMap docMap, Sort sort) {
     LeafMetaData metaData = reader.getMetaData();
     LeafMetaData newMetaData =
         new LeafMetaData(metaData.getCreatedVersionMajor(), metaData.getMinVersion(), sort);

lucene/core/src/java/org/apache/lucene/util/OfflineSorter.java

@@ -561,7 +561,7 @@ public static class ByteSequencesReader implements BytesRefIterator, Closeable {
     protected final String name;
     protected final ChecksumIndexInput in;
     protected final long end;
-    private final BytesRefBuilder ref = new BytesRefBuilder();
+    protected final BytesRefBuilder ref = new BytesRefBuilder();
 
     /** Constructs a ByteSequencesReader from the provided IndexInput */
     public ByteSequencesReader(ChecksumIndexInput in, String name) {