Move some code into models/git (#19879)

* Move access and repo permission to models/perm/access

* fix test

* Move some git related files into sub package models/git

* Fix build

* fix git test

* move lfs to sub package

* move more git related functions to models/git

* Move functions sequence

* Some improvements per @KN4CK3R and @delvh

models/git/main_test.go

@@ -0,0 +1,18 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package git_test
+
+import (
+	"path/filepath"
+	"testing"
+
+	"code.gitea.io/gitea/models/unittest"
+)
+
+func TestMain(m *testing.M) {
+	unittest.MainTest(m, &unittest.TestOptions{
+		GiteaRootPath: filepath.Join("..", ".."),
+	})
+}