Add Pull Request merge options - Ignore white-space for conflict checking, Rebase, Squash merge (#3188)

* Pull request options migration and UI in settings

* Add ignore whitespace functionality

* Fix settings if pull requests are disabled

* Fix migration transaction

* Merge with Rebase functionality

* UI changes and related functionality for pull request merging button

* Implement squash functionality

* Fix rebase merging

* Fix pull request merge tests

* Add squash and rebase tests

* Fix API method to reuse default message functions

* Some refactoring and small fixes

* Remove more hardcoded values from tests

* Remove unneeded check from API method

* Fix variable name and comment typo

* Fix reset commit count after PR merge

models/migrations/v54.go

@@ -0,0 +1,57 @@
+// Copyright 2017 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 (
+	"fmt"
+
+	"code.gitea.io/gitea/modules/util"
+
+	"github.com/go-xorm/xorm"
+)
+
+func addPullRequestOptions(x *xorm.Engine) error {
+	// RepoUnit describes all units of a repository
+	type RepoUnit struct {
+		ID          int64
+		RepoID      int64                  `xorm:"INDEX(s)"`
+		Type        int                    `xorm:"INDEX(s)"`
+		Config      map[string]interface{} `xorm:"JSON"`
+		CreatedUnix util.TimeStamp         `xorm:"INDEX CREATED"`
+	}
+
+	sess := x.NewSession()
+	defer sess.Close()
+	if err := sess.Begin(); err != nil {
+		return err
+	}
+
+	//Updating existing issue units
+	units := make([]*RepoUnit, 0, 100)
+	if err := sess.Where("`type` = ?", V16UnitTypePRs).Find(&units); err != nil {
+		return fmt.Errorf("Query repo units: %v", err)
+	}
+	for _, unit := range units {
+		if unit.Config == nil {
+			unit.Config = make(map[string]interface{})
+		}
+		if _, ok := unit.Config["IgnoreWhitespaceConflicts"]; !ok {
+			unit.Config["IgnoreWhitespaceConflicts"] = false
+		}
+		if _, ok := unit.Config["AllowMerge"]; !ok {
+			unit.Config["AllowMerge"] = true
+		}
+		if _, ok := unit.Config["AllowRebase"]; !ok {
+			unit.Config["AllowRebase"] = true
+		}
+		if _, ok := unit.Config["AllowSquash"]; !ok {
+			unit.Config["AllowSquash"] = true
+		}
+		if _, err := sess.ID(unit.ID).Cols("config").Update(unit); err != nil {
+			return err
+		}
+	}
+	return sess.Commit()
+}