Fix rollback by re-adding baseRevIdForSaving in EntityContent

This is an ugly hack and should be made nicer.

This code is interacting with complex core code and is
needed as a workaround for now.

This patch essentially restores functionality that was
removed in Ia99a5f29.

Bug: 65831
Change-Id: If1e7db531f6a8fa3e76edbef26d1e344f7178218