Add a simple way to rename branch like gh (#15870)

- Update default branch if needed
- Update protected branch if needed
- Update all not merged pull request base branch name
- Rename git branch
- Record this rename work and auto redirect for old branch on ui

Signed-off-by: a1012112796 <1012112796@qq.com>
Co-authored-by: delvh <dev.lh@web.de>

models/migrations/v197.go

@@ -0,0 +1,20 @@
+// 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 addRenamedBranchTable(x *xorm.Engine) error {
+	type RenamedBranch struct {
+		ID          int64 `xorm:"pk autoincr"`
+		RepoID      int64 `xorm:"INDEX NOT NULL"`
+		From        string
+		To          string
+		CreatedUnix int64 `xorm:"created"`
+	}
+	return x.Sync2(new(RenamedBranch))
+}