clean: integrate with sparse index

clean: integrate with sparse index

Remove full index requirement for `git clean` and test to ensure the index
is not expanded in `git clean`. Add to existing test for `git clean` to
verify cleanup of untracked files in sparse directories is consistent
between sparse index and non-sparse index checkouts.

Signed-off-by: Victoria Dye <redacted>
Reviewed-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>