Repairs the index using previously returned result from
CheckIndex. Note that this does not remove any of the unreferenced files after it's done; you must separately open an
IndexWriter, which deletes unreferenced files when it's created.
WARNING: this writes a new segments file into the index, effectively removing all documents in broken segments from the index. BE CAREFUL.
WARNING: Make sure you only call this when the index is not opened by any writer.