don't let new merges start after finishMerges when closing

git-svn-id: https://svn.apache.org/repos/asf/lucene/java/trunk@776877 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/lucene/index/IndexWriter.java

@@ -2069,6 +2069,7 @@ public class IndexWriter {
       mergePolicy.close();
 
       finishMerges(waitForMerges);
+      stopMerges = true;
 
       mergeScheduler.close();