Merge changes I853ac6c7,I01878116,Ie994fc18 into stable-5.1

* changes:
  BatchRefUpdate: Skip saving conflicting ref names and prefixes in memory
  BatchRefUpdateTest: Accurately assert RefsChangedEvent(s) fired
  Optimize RefDirectory.isNameConflicting()

.bazelversion

diff --git a/.bazelversion b/.bazelversion
index 3eefcb9..227cea2 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-1.0.0
+2.0.0

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 0ddbf8f..46cb265 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -15,7 +15,7 @@
 
 load("//tools:bazlets.bzl", "load_bazlets")
 
-load_bazlets(commit = "09a035e98077dce549d5f6a7472d06c4b8f792d2")
+load_bazlets(commit = "f53f51fb660552d0581aa0ba52c3836ed63d56a3")
 
 load(
     "@com_googlesource_gerrit_bazlets//tools:maven_jar.bzl",