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)

modelmigration/base/git.go

@@ -0,0 +1,13 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package base
+
+import (
+	repo_model "gitea.dev/models/repo"
+	"gitea.dev/modules/git/gitcmd"
+)
+
+func LocalCodeGitRepo(ownerName, repoName string) gitcmd.RepositoryFacade {
+	return repo_model.CodeRepoByName(ownerName, repoName)
+}