Merge branch 'stable-3.1'

* stable-3.1:
  Upgrade bazlets to latest stable-3.1 to build with 3.1.7 API
  CreateChangesTriggeringGc: Support existing project
  Upgrade bazlets to latest stable-3.0 to build with 3.0.11 API
  Remove the commented-out snapshot plugin api lines
  Adapt SNAPSHOT plugin api example to the 3.1 version
  Upgrade bazlets to latest stable-3.1
  e2e Scenarios: Refine the gc checking resolution
  Prepare for new gerrit_api snapshot version usage
  Upgrade bazlets to latest stable-3.0
  Upgrade bazlets to latest stable-3.0
  CheckProjectStatisticsAfterGc: Make tests circular

Change-Id: I912453b22511f9ab395b50e6cd52befde2eb1701