Write an info to the log when a change gets deleted from the index

This is helpful to investigate issues where a change is unexpectedly
deleted from the index.

Change-Id: Ic9d559c6f72fcc36b1c13e32fde95b68a6ef4de3

gerrit-server/src/main/java/com/google/gerrit/server/index/change/ChangeIndexer.java

@@ -337,6 +337,7 @@ public class ChangeIndexer {
       for (ChangeIndex i : getWriteIndexes()) {
         i.delete(id);
       }
+      log.info("Deleted change {} from index.", id.get());
       fireChangeDeletedFromIndexEvent(id.get());
       return null;
     }