Use OldRef instead of CommitSHA for DeleteBranch comments (#14604)

Fix #14545

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

models/migrations/v169.go

@@ -0,0 +1,14 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package migrations
+
+import (
+	"xorm.io/xorm"
+)
+
+func commentTypeDeleteBranchUseOldRef(x *xorm.Engine) error {
+	_, err := x.Exec("UPDATE comment SET old_ref = commit_sha, commit_sha = '' WHERE type = 11")
+	return err
+}