Fix is similar to how CollectionObject with NULL object pointers are handled.
Using one of the 'free' pad bytes in Object_Runtime struct instead of a
gset (or other external way to detect object duplicates), as this is
several times faster.
NOTE: This makes remapping slightly slower again (adds 10 extra seconds
to file case in T94059).
General improvements of remapping time complexity, especially when
remapping a lot of IDs at once, is a separate topic currently
investigated in D13615.
8d3e57f338