feat!: use del() function instead of assigning null for ignoring member

cmd/jsondiff/testdata/ok_with_ignore_option.stdout.txt

@@ -1,12 +1,11 @@
 --- from.json
 +++ to.json
-@@ -1,7 +1,7 @@
+@@ -1,6 +1,6 @@
  {
    "a": 1,
 -  "b": 2,
--  "c": 3,
+-  "c": 3
 +  "b": 1,
-+  "c": 2,
-   "d": null
++  "c": 2
  }
 

diff.go

@@ -103,7 +103,7 @@ func Diff(from, to *Input, opts ...Option) (string, error) {
 	switch {
 	case o.ignore != nil:
 		var err error
-		q := WithUpdate(o.ignore)
+		q := deleting(o.ignore)
 		fromObj, err = ModifyValue(q, fromObj)
 		if err != nil {
 			return "", fmt.Errorf("modify(lhs): %v", err)
@@ -186,6 +186,18 @@ func WithUpdate(query *gojq.Query) *gojq.Query {
 	return ret
 }
 
+func deleting(query *gojq.Query) *gojq.Query {
+	return &gojq.Query{
+		Term: &gojq.Term{
+			Type: gojq.TermTypeFunc,
+			Func: &gojq.Func{
+				Name: "del",
+				Args: []*gojq.Query{query},
+			},
+		},
+	}
+}
+
 var nullRhs *gojq.Query
 
 func init() {

diff_test.go

@@ -72,6 +72,28 @@ func Test_withUpdate(t *testing.T) {
 	}
 }
 
+func Test_deleting(t *testing.T) {
+	queries := []struct {
+		query string
+		want  string
+	}{
+		{".age", "del(.age)"},
+		{".age, .name", "del(.age, .name)"},
+		{".age, .name, .meta", "del(.age, .name, .meta)"},
+		{".meta[]", "del(.meta[])"},
+		{".meta[0:-1]", "del(.meta[0:-1])"},
+	}
+	for _, c := range queries {
+		t.Run(c.query, func(t *testing.T) {
+			want := parseQuery(t, c.want)
+			got := deleting(parseQuery(t, c.query))
+			if diff := cmp.Diff(want, got); diff != "" {
+				t.Errorf("-want, +got:\n%s", diff)
+			}
+		})
+	}
+}
+
 func parseQuery(t *testing.T, q string) *gojq.Query {
 	t.Helper()
 	parsed, err := gojq.Parse(q)

example_test.go

@@ -69,10 +69,9 @@ func ExampleDiffFromObjects_ignore() {
 	// Output:
 	// --- from
 	// +++ to
-	// @@ -2,6 +2,6 @@
+	// @@ -1,5 +1,5 @@
+	//  {
 	//    "a": 1,
-	//    "b": null,
-	//    "c": null,
 	// -  "d": 4
 	// +  "d": 3
 	//  }