Merge branch 'stable-2.15'

Prefer the JGit version from master, which is based off of JGit master
and contains the same commits as in the stable branch.

* stable-2.15:
  SetReadyForReview: Change action label to "Start Review"
  Tighten vertical spacing in messages
  NoteDbMigrator: Use new pack-based inserter implementation
  Upgrade JGit to 4.9.0.201710071750-r.8-g678c99c05
  NoteDbMigrator: Use one NoteDbUpdateManager per project
  LockFailureException: Expose ref names
  ChangeRebuilderImpl: Handle createdOn > lastUpdatedOn

Change-Id: Ic842d494fff7641de24ebd15ca33e4e91a5c400d

java/com/google/gerrit/server/notedb/ChangeNotes.java

@@ -779,6 +779,7 @@ public class ChangeNotes extends AbstractChangeNotes<ChangeNotes> {
           rebuildResult = checkNotNull(r);
           checkNotNull(r.newState());
           checkNotNull(r.staged());
+          checkNotNull(r.staged().changeObjects());
           return LoadHandle.create(
               ChangeNotesCommit.newStagedRevWalk(repo, r.staged().changeObjects()),
               r.newState().getChangeMetaId());