tweak SortingMergePolicy.getSortDescription

Building tests for SOLR-5730 identified that early termination can be omitted for sorted segments because the LeafReader concerned is not a SegmentReader - github pull request to illustrate to follow:

• the TestEarlyTerminatingSortingCollector.testTerminatedEarly test uses a wrapped reader in the same way as SolrIndexSearcher
• the SortingMergePolicy.getSortDescription change (assuming it is a valid change to make) fixes that particular test only
• ExitableDirectoryReader and UninvertingReader wrap could perhaps also be added in LuceneTestCase.wrapReader ?

LUCENE-6065 "remove "foreign readers" from merge, fix LeafReader instead." also concerns SortingMergePolicy.