sparse-checkout: provide a new reapply subcommand

sparse-checkout: provide a new reapply subcommand

If commands like merge or rebase materialize files as part of their work,
or a previous sparse-checkout command failed to update individual files
due to dirty changes, users may want a command to simply 'reapply' the
sparsity rules.  Provide one.

Reviewed-by: Derrick Stolee <redacted>
Signed-off-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>