Issue exposed by rB4c7b1766a7f1.
Main idea is that non-memfile first undo step should check into previous
memfile and tag the ID it is editing as `future_changed`.
That way, when we go back and undo to the memfile, said IDs are properly
detected as changed and re-read from the memfile.
Otherwise, undo system sees them as unchanged, and just re-use the
current data instead.
Note that currently only Sculpt mode seems affected (probably because it
is storing the mode switch itself as a Sculpt undo step instead of a
memfile one), but similar action might be needed in some other cases
too.
Maniphest Tasks: T82388
Differential Revision: https://developer.blender.org/D9510
bc090387ac