* stable-2.15:
ChangeApi: Add default implementation of setPrivate with null message
ChangeApi: Annotate nullable message parameters as @Nullable
ProjectApi: Add methods to get/set project's HEAD
CommitApi: Add method to get "included in" information
Upgrade elasticsearch-rest-client to 6.5.4
Change-Id: I25b8ad37fbca65235d9139d38d6d7bcdd0ff6e11