There's no reason not to allow these.
Technically we could enable a few more, but I chose not to because some might be
surprising or confusing in filtering mode. For example, creating a fixup commit
would work (shift-F), but the newly created commit might not show up if it
doesn't match the filter. Similarly, pressing `f` to fixup a commit into its
parent would work, but that parent commit might not be visible, so users might
expect to be fixing up into the next visible commit.
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