[FEAT] add Forgejo Git Service (squash) register a Forgejo factory

If the Forgejo factory for the Forgejo service is not registered,
newDownloader will fallback to a git service and not migrate issues
etc.

Refs: https://codeberg.org/forgejo/forgejo/issues/1678

services/migrations/forgejo_downloader.go

@@ -0,0 +1,20 @@
+// Copyright 2023 The Forgejo Authors
+// SPDX-License-Identifier: MIT
+
+package migrations
+
+import (
+	"code.gitea.io/gitea/modules/structs"
+)
+
+func init() {
+	RegisterDownloaderFactory(&ForgejoDownloaderFactory{})
+}
+
+type ForgejoDownloaderFactory struct {
+	GiteaDownloaderFactory
+}
+
+func (f *ForgejoDownloaderFactory) GitServiceType() structs.GitServiceType {
+	return structs.ForgejoService
+}

services/migrations/forgejo_downloader_test.go

@@ -0,0 +1,16 @@
+// Copyright 2023 The Forgejo Authors
+// SPDX-License-Identifier: MIT
+
+package migrations
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/modules/structs"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestForgejoDownload(t *testing.T) {
+	require.NotNil(t, getFactoryFromServiceType(structs.ForgejoService))
+}

services/migrations/migrate.go

@@ -20,6 +20,7 @@ import (
 	"code.gitea.io/gitea/modules/log"
 	base "code.gitea.io/gitea/modules/migration"
 	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/structs"
 	"code.gitea.io/gitea/modules/util"
 )
 
@@ -139,20 +140,26 @@ func MigrateRepository(ctx context.Context, doer *user_model.User, ownerName str
 	return uploader.repo, nil
 }
 
+func getFactoryFromServiceType(serviceType structs.GitServiceType) base.DownloaderFactory {
+	for _, factory := range factories {
+		if factory.GitServiceType() == serviceType {
+			return factory
+		}
+	}
+	return nil
+}
+
 func newDownloader(ctx context.Context, ownerName string, opts base.MigrateOptions) (base.Downloader, error) {
 	var (
 		downloader base.Downloader
 		err        error
 	)
 
-	for _, factory := range factories {
+	if factory := getFactoryFromServiceType(opts.GitServiceType); factory != nil {
-		if factory.GitServiceType() == opts.GitServiceType {
 		downloader, err = factory.New(ctx, opts)
 		if err != nil {
 			return nil, err
 		}
-			break
-		}
 	}
 
 	if downloader == nil {