gerritapi: show ReopenedEvent when CL is restored

When a Gerrit CL change is restored, there should be an
associated ReopenedEvent. This change implements that.

I've finally encountered a Gerrit CL that was restored,
so this change could be tested on it:
dmitshur committed 1 year ago commit e9885884f07001d494be49738df5397e05019c5a
@@ -340,10 +340,16 @@ func (s service) ListTimeline(ctx context.Context, repo string, id uint64, opt *
 					User:      s.gerritUser(message.Author),
 					CreatedAt: message.Date.Time,
 					Body:      strings.TrimPrefix(message.Message, "Abandoned\n\n"),
 					Editable:  false,
+			case "gerrit:restore":
+				timeline = append(timeline, change.TimelineItem{
+					Actor:     s.gerritUser(message.Author),
+					CreatedAt: message.Date.Time,
+					Payload:   change.ReopenedEvent{},
+				})
 		labels, body, ok := parseMessage(message.Message)
 		if !ok {