LUCENE-4898: openIfChange now allows targeting an IndexCommit from an NRT reader

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1464437 13f79535-47bb-0310-9956-ffa450edef68
move package line to top

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1464471 13f79535-47bb-0310-9956-ffa450edef68