Adapt diff-root script to work with presampled pile-up RDOs

The container names in presampled pile-up RDOs differ from standard
RDO files. In order for the `--order-trees` option to work, then the
script has to be made aware of `xAOD::EventAuxInfo_v2_Bkg_EventInfoAux.`
rather than just looking for the standard container name.