Automerge supports deleting branch automatically after merging (#32343)

Resolve #32341 
~Depends on #27151~

- [x] It will display a checkbox of deleting the head branch on the pull
request view page when starting an auto-merge task.
- [x] Add permission check before deleting the branch
- [x] Add delete branch comment for those closing pull requests because
of head branch or base branch was deleted.
- [x] Merge `RetargetChildrenOnMerge` and `AddDeletePRBranchComment`
into `service.DeleteBranch`.

models/migrations/migrations.go

@@ -22,6 +22,7 @@ import (
 	"code.gitea.io/gitea/models/migrations/v1_21"
 	"code.gitea.io/gitea/models/migrations/v1_22"
 	"code.gitea.io/gitea/models/migrations/v1_23"
+	"code.gitea.io/gitea/models/migrations/v1_24"
 	"code.gitea.io/gitea/models/migrations/v1_6"
 	"code.gitea.io/gitea/models/migrations/v1_7"
 	"code.gitea.io/gitea/models/migrations/v1_8"
@@ -369,6 +370,9 @@ func prepareMigrationTasks() []*migration {
 		newMigration(309, "Improve Notification table indices", v1_23.ImproveNotificationTableIndices),
 		newMigration(310, "Add Priority to ProtectedBranch", v1_23.AddPriorityToProtectedBranch),
 		newMigration(311, "Add TimeEstimate to Issue table", v1_23.AddTimeEstimateColumnToIssueTable),
+
+		// Gitea 1.23.0-rc0 ends at migration ID number 311 (database version 312)
+		newMigration(312, "Add DeleteBranchAfterMerge to AutoMerge", v1_24.AddDeleteBranchAfterMergeForAutoMerge),
 	}
 	return preparedMigrations
 }

models/migrations/v1_24/v312.go

@@ -0,0 +1,21 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_24 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+type pullAutoMerge struct {
+	DeleteBranchAfterMerge bool
+}
+
+// TableName return database table name for xorm
+func (pullAutoMerge) TableName() string {
+	return "pull_auto_merge"
+}
+
+func AddDeleteBranchAfterMergeForAutoMerge(x *xorm.Engine) error {
+	return x.Sync(new(pullAutoMerge))
+}