Add repo upgrade migration

internal/migrations/upgrade_repo_v2.go

@@ -0,0 +1,46 @@
+package migrations
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/restic/restic/internal/restic"
+)
+
+func init() {
+	register(&UpgradeRepoV2{})
+}
+
+type UpgradeRepoV2 struct{}
+
+func (*UpgradeRepoV2) Name() string {
+	return "upgrade_repo_v2"
+}
+
+func (*UpgradeRepoV2) Desc() string {
+	return "upgrade a repository to version 2"
+}
+
+func (*UpgradeRepoV2) Check(ctx context.Context, repo restic.Repository) (bool, error) {
+	isV1 := repo.Config().Version == 1
+	return isV1, nil
+}
+
+func (*UpgradeRepoV2) Apply(ctx context.Context, repo restic.Repository) error {
+	cfg := repo.Config()
+	cfg.Version = 2
+
+	h := restic.Handle{Type: restic.ConfigFile}
+
+	err := repo.Backend().Remove(ctx, h)
+	if err != nil {
+		return fmt.Errorf("remove old config file failed: %w", err)
+	}
+
+	_, err = repo.SaveJSONUnpacked(ctx, restic.ConfigFile, cfg)
+	if err != nil {
+		return fmt.Errorf("save new config file failed: %w", err)
+	}
+
+	return nil
+}