RepoOnlyOps is for operations that not tied to a Change, usually, the
operations that only touch the repository, e.g., commit created
by Gerrit.
For example, during submodule submission, there are two changes in the
same topic: one change is for the master branch of superproject A, the
other is for the master branch of a submodule A'. At the same time, dev
branch of A is also subscribed to master branch of A'. When we submit
the whole topic together, we need update master branch of A and A' by
SubmitStrategyOp, also the dev branch of A with the gitlink commit by
this RepoOnlyOp. And group all the Ops for superproject A in a single
BatchUpdate.
Change-Id: I18b391f5c858b8608745435d465a63596b3eccac
4c8142df0d