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/repostats/repo_test.go

@@ -0,0 +1,37 @@
+// Copyright 2017 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repostats
+
+import (
+	"testing"
+
+	issues_model "gitea.dev/models/issues"
+	"gitea.dev/models/unittest"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCheckRepoStats(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+	assert.NoError(t, CheckRepoStats(t.Context()))
+}
+
+func TestDoctorUserStarNum(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	assert.NoError(t, DoctorUserStarNum(t.Context()))
+}
+
+func Test_repoStatsCorrectIssueNumComments(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	issue2 := unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{ID: 2})
+	assert.NotNil(t, issue2)
+	assert.Equal(t, 0, issue2.NumComments) // the fixture data is wrong, but we don't fix it here
+
+	assert.NoError(t, repoStatsCorrectIssueNumComments(t.Context(), 2))
+	// reload the issue
+	issue2 = unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{ID: 2})
+	assert.Equal(t, 1, issue2.NumComments)
+}