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
diff --git a/WORKSPACE b/WORKSPACE
index be398a1..ea097a8 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "df6622a4e725b4d0bad99377f05749171a8c94b9",
+    commit = "ad9df4d323c1df5c733d379c8891bde13a00a285",
     #    local_path = "/home/<user>/projects/bazlets",
 )