sparse-checkout: add sanity-checks on initial sparsity state

Most sparse-checkout subcommands (list, add, reapply) only make sense
when already in a sparse state.  Add a quick check that will error out
early if this is not the case.

Also document with a comment why we do not exit early in `disable` even
when core.sparseCheckout starts as false.

Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Victoria Dye <vdye@github.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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