Make all entity object instantiations pass through VersioncontrolBackend::buildEntity()

With the entities changeover, _all_ entity objects should be created via a call to VersioncontrolBackend::buildObject(), as the backends now act as central factories with mappings between a string 'type' (e.g., "branch", "repository") and the actual class to be used ("VersioncontrolGitBranch", "VersioncontrolGitRepository"). The factory in the backend class is likely to get more complicated as time goes on, so all the more important that we make sure we run all our logic through there.