Fix bug on pull requests when transfer head repository (#8564) (#8569)

* fix bug on pull requests when transfer head repository

* add migration and fix lint

* fix tests and add a cache check on LoadBaseRepo

models/migrations/v102.go

@@ -0,0 +1,15 @@
+// Copyright 2019 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 (
+	"github.com/go-xorm/xorm"
+)
+
+func dropColumnHeadUserNameOnPullRequest(x *xorm.Engine) error {
+	sess := x.NewSession()
+	defer sess.Close()
+	return dropTableColumns(sess, "pull_request", "head_user_name")
+}