refactor: use WithRepo instead of WithDir for most git operations, clean up model migrations (#38555)

by the way, remove some unnecessary "models" imports from model
migration package, fix migration test model init bug
(modelmigration/migrationtest/tests.go)

models/init.go

@@ -0,0 +1,15 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package models
+
+import (
+	"context"
+
+	"gitea.dev/models/unit"
+)
+
+// Init initialize model
+func Init(ctx context.Context) error {
+	return unit.LoadUnitConfig()
+}