History diff: keep a stepped-back object in place; number hunks right

Rewind put an object's old form at the end of its list, so a history
entry's diff showed the changed object moving (turning hairpin off for
one forward also moved its forward and DNAT lines). It now sorts after
stepping back. go-udiff v0.4.1 numbers the new side of hunks after one
with a moved line too early, so the chain after @@ could be the one
before; hunkStarts numbers them again. History diffs name their sides
"before N" and "after N".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/diff_test.go

@@ -1,6 +1,8 @@
 package server
 
 import (
+	"encoding/json"
+	"fmt"
 	"strings"
 	"testing"
 
@@ -70,3 +72,68 @@ func TestHistoryDiff(t *testing.T) {
 		t.Errorf("unknown entry: %d", code)
 	}
 }
+
+// TestHunkStarts: after a hunk with unchanged lines between its edits (a
+// moved line), the later hunks' new sides start where they do.
+func TestHunkStarts(t *testing.T) {
+	var a, b strings.Builder
+	for i := 1; i <= 20; i++ {
+		fmt.Fprintf(&a, "%d\n", i)
+		switch i {
+		case 4:
+		case 6:
+			b.WriteString("6\n4\n")
+		case 14:
+		default:
+			fmt.Fprintf(&b, "%d\n", i)
+		}
+	}
+	diff := hunkStarts(udiff.Unified("a", "b", a.String(), b.String()))
+	var heads []string
+	for _, l := range strings.Split(diff, "\n") {
+		if strings.HasPrefix(l, "@@") {
+			heads = append(heads, l)
+		}
+	}
+	// diff -u: the move nets nothing, the deleted 14 one line.
+	if want := []string{"@@ -1,9 +1,9 @@", "@@ -11,7 +11,6 @@"}; strings.Join(heads, "|") != strings.Join(want, "|") {
+		t.Fatalf("hunk headers %q, want %q\n%s", heads, want, diff)
+	}
+}
+
+// TestHistoryDiffUpdate: an update's diff is only what it changed; the
+// object stepped back to its old form stays where it was.
+func TestHistoryDiffUpdate(t *testing.T) {
+	e := newEnv(t)
+	e.call(e.token, "POST", "/zones", model.Zone{Name: "lan", Interfaces: []string{"eth1"}}, nil)
+	e.call(e.token, "POST", "/zones", model.Zone{Name: "wan", Interfaces: []string{"wan"}, External: true}, nil)
+	var ids []int64
+	for i, port := range []string{"8096", "443", "50300", "7777", "25"} {
+		var mr api.MutationResponse
+		e.call(e.token, "POST", "/forwards", map[string]any{"name": "f" + port, "ports": port, "to": fmt.Sprintf("10.0.10.%d", 30+i),
+			"public_addresses": []string{"203.0.113.1"}, "hairpin": true}, &mr)
+		var f model.PortForward
+		if err := json.Unmarshal(mr.Object, &f); err != nil {
+			t.Fatal(err)
+		}
+		ids = append(ids, f.ID)
+	}
+	var mr api.MutationResponse
+	if code := e.call(e.token, "PATCH", "/forwards/"+itoa(ids[2]), map[string]any{"hairpin": false}, &mr); code != 200 {
+		t.Fatalf("update: %d", code)
+	}
+	var out struct{ Diff string }
+	if code := e.call(e.token, "GET", "/history/"+itoa(mr.HistoryID)+"/diff", nil, &out); code != 200 {
+		t.Fatalf("diff: %d", code)
+	}
+	var changed []string
+	for _, l := range strings.Split(out.Diff, "\n") {
+		if (strings.HasPrefix(l, "+") || strings.HasPrefix(l, "-")) && !strings.HasPrefix(l, "+++") && !strings.HasPrefix(l, "---") {
+			changed = append(changed, l)
+		}
+	}
+	if len(changed) != 1 || !strings.HasPrefix(changed[0], "-") || !strings.Contains(changed[0], "-hairpin") ||
+		!strings.Contains(out.Diff, "(before "+itoa(mr.HistoryID)+")") || !strings.Contains(out.Diff, "@@ chain srcnat") {
+		t.Errorf("entry %d's diff, want only the hairpin line gone, in chain srcnat:\n%s", mr.HistoryID, out.Diff)
+	}
+}