Update bazlets to latest revision on stable-2.15
Update bazlets to latest revision on stable-2.15

- Add optional dir_name argument to gerrit_plugin
- gerrit_plugin: Improve stamping error message
- Upgrade Gerrit API to version 2.15.2
- Merge branch 'stable-2.14' into stable-2.15
- Set Gerrit API snapshot version to 2.15.2-SNAPSHOT

Change-Id: Ie7c36939af9ae885df34d7631494d16b22de37af

GerritCodeReview/plugins_reviewers,GerritCodeReview/plugins_reviewers,GerritCodeReview/plugins_reviewers