Merge branch 'stable-5.6' into stable-5.7

* stable-5.6:
  Prepare 5.1.17-SNAPSHOT builds
  JGit v5.1.16.202106041830-r
  BatchRefUpdate: Skip saving conflicting ref names and prefixes in memory
  BatchRefUpdateTest: Accurately assert RefsChangedEvent(s) fired
  Optimize RefDirectory.isNameConflicting()
  Update bazlets and bazel version

Change-Id: I4570cce185877cb4c50eee519a1cf9467a766dea

diff --cc org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/BatchRefUpdateTest.java
Simple merge

diff --cc org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/RefDirectory.java
Simple merge

diff --cc org.eclipse.jgit/src/org/eclipse/jgit/lib/BatchRefUpdate.java
Simple merge