Embedded IDs never get tagged as 'unchanged', since this status is
always the same as for their owner ID.
So far, this assert was never reached, which would indicate that IDs
using embedded data are never detected as unchanged. Hopefully work like
the 'pointer stability' on blendfile writing (!127729) will improve this
situation.
The problem was triggered in upcoming packed data code, where packed IDs
are managed like regular local ones on memfile undo (and not like
regular linked ones). But in that case it looks like packed materials
and such are properly detected as unchanged...
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