sparse-checkout: break apart functions for sparse_checkout_(set|add)

sparse-checkout: break apart functions for sparse_checkout_(set|add)

sparse_checkout_set() was reused by sparse_checkout_add() with the only
difference being a single parameter being passed to that function.
However, we would like sparse_checkout_set() to do the same work that
sparse_checkout_init() does if sparse checkouts are not already enabled.
To facilitate this transition, give each mode their own copy of the
function.  This does not introduce any behavioral changes; that will
come in a subsequent patch.

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>