Rename prepareSubscriptionConfigEntry to prepareSubmoduleConfigEntry,
and make the subscribeToBranch parameter optional. When null is given,
the "branch" value is not added in the config, which means that the
submodule should be manually updated rather than being automatically
updated by the subscription.
This will make it possible to reuse these methods for tests that need
to add/update submodules.
Change-Id: Ib3b18e05daecff885b19810272e6fa836914a696
a8f17980c7