clone: avoid using deprecated `sparse-checkout init`

The previous commits marked `sparse-checkout init` as deprecated; we
can just use `set` instead here and pass it no paths.

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>

builtin/clone.c

@@ -633,7 +633,7 @@ static int git_sparse_checkout_init(const char *repo)
 {
 	struct strvec argv = STRVEC_INIT;
 	int result = 0;
-	strvec_pushl(&argv, "-C", repo, "sparse-checkout", "init", NULL);
+	strvec_pushl(&argv, "-C", repo, "sparse-checkout", "set", NULL);
 
 	/*
 	 * We must apply the setting in the current process