Store topic of a change in Notedb

This adds a 'Topic' footer to  the commit messages of the
refs/changes/XX/YYYY/meta notes branch when the topic changes.

Similar as for hashtags we cannot rebuild the history for topic
changes since the database only stores the current topic.

Change-Id: I5ca2c1ed6326ba03dfbee21f1a7d2328b73ad322
Signed-off-by: Edwin Kempin <ekempin@google.com>

gerrit-server/src/main/java/com/google/gerrit/server/change/ChangeInserter.java

@@ -250,6 +250,7 @@ public class ChangeInserter extends BatchUpdate.InsertChangeOp {
     }
     db.patchSets().insert(Collections.singleton(patchSet));
     db.changes().insert(Collections.singleton(change));
+    update.setTopic(change.getTopic());
     LabelTypes labelTypes = ctl.getProjectControl().getLabelTypes();
     approvalsUtil.addReviewers(db, update, labelTypes, change,
         patchSet, patchSetInfo, reviewers, Collections.<Account.Id> emptySet());