checkout-index: improve ensure_full_index scope in checkout_all

Update `checkout_all` to only run `ensure_full_index` when entries in
sparse directories are needed (i.e., `--sparse` is specified).

Signed-off-by: Victoria Dye <vdye@github.com>

builtin/checkout-index.c

@@ -122,8 +122,9 @@ static int checkout_all(const char *prefix, int prefix_length, int include_spars
 	int i, errs = 0;
 	struct cache_entry *last_ce = NULL;
 
-	/* TODO: audit for interaction with sparse-index. */
-	ensure_full_index(&the_index);
+	if (include_sparse)
+		ensure_full_index(&the_index);
+
 	for (i = 0; i < active_nr ; i++) {
 		struct cache_entry *ce = active_cache[i];
 		if (!include_sparse && !path_in_sparse_checkout(ce->name, &the_index))