TEvent::fill() Fix, master branch (2020.10.20.)
This is a manual cherry-pick of !37405 (merged). (The copyright messages at the top of TEvent.cxx
differ between 21.2 and master. That caused an issue for the automatic cherry-pick...)
As described in the original MR, this is meant to fix ATLASG-1555. An issue with xAOD::TEvent::fill()
not writing out all dynamic branches of a container correctly. As found by @robouque.