Merge branch 'stable-2.15' into master

Fix two test failures due to the merge:

* StandaloneNoteDbMigrationIT assumed that GCing All-Users would prune
  all loose objects, whereas in recent JGit versions, unreachable
  objects are left loose unless gc.pruneExpire is set to "now". Do this
  in the tests.

* AbstractQueryChangesTest#watched modifies the user's project watches.
  In the master branch, watches are now stored in the AccountState
  instance, so we need to recreate the IdentifiedUser in order to reload
  the AccountState.

* origin/stable-2.15: (33 commits)
  Release 2.15
  Mention groups index in documentation of index start command
  Enable UI action to toggle WIP flag for admins
  Docs: Clarify that for external groups the name in GroupInfo can be missing
  AccountGroupAuditLogScreen: Display group UUID if group name is missing
  GetAuditLog: Fix NPE if group UUID cannot be resolved
  Limit assignee suggestions to users that can see the change
  Add account predicate that checks if user can see a certain change
  Allow admins to toggle the WIP flag on all changes
  Included-In dialog polish
  PolyGerrit: Add support for "Included In"
  Make template testing faster
  Limit assignee suggestions to users that can see the change
  Add account predicate that checks if user can see a certain change
  AbstractQueryChangesTest: Extend byDraftBy to include test for "has:draft"
  AbstractQueryChangesTest: Add explicit tests for is:watched and watchedby:
  AbstractQueryChangesTest: Add explicit tests for is:abandoned and status:abandoned
  user-search: Clarify behavior of default search resulting in single change
  user-search: Fix query used in "My > Watched Changes"
  Documentation: Clarify ref-updated event content when ref is deleted
  Update git submodules
  Fix markup in Documentation section sendemail
  Allow graceful rolling restarts
  Release 2.15-rc4
  user-search: Remove incorrect statement about default searches
  user-search: Fix incorrect label: search example
  StandaloneSiteTest: Ignore user and system git config
  Fix documentation about create and delete branches
  GcAllUsers: Respect gc.auto=0 in All-Users
  GcAllUsers: Add extra log line in online case
  MigrateToNoteDb: Auto-flush and close GC writer
  Turn off autoReindexIfStale by default
  GC All-Users after migrating to NoteDb

Change-Id: If2dff6bd1e0b299be75878aca3bc45103a5a875f