* stable-2.15:
ReceiveCommits: Validate ref operation before commits for non-ff case
Add test coverage for ref operation validation extension point
Upgrade JGit to 4.11.8.201904181247-r
CreateProject: Expose createProject method
ElasticContainer: Switch to Alpine-based images from blacktop
Bazel: Fix lint warning flagged by buildifier
Change-Id: I43f6b82744788ffe6635abc3c06af57cf0629a5e