Refuse to create instant fixup commit for an unreachable commit
When performing an instant fixup it is clearly a mistake to select an unreachable commit. The second step, the rebase, would always fail. However creating a fixup commit for an unreachable commit without trying to instantly rebase is a valid use-case, it allows "labeling" the fixup as belonging to some commit. Just doing a rebase would not be enough to get the fixup to the right place, but we cannot know whether the user is aware of that - so continue to allow the creation of such fixup commits.
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