Refactor Git Attribute & performance optimization (#34154)

This PR moved git attributes related code to `modules/git/attribute` sub
package and moved language stats related code to
`modules/git/languagestats` sub package to make it easier to maintain.

And it also introduced a performance improvement which use the `git
check-attr --source` which can be run in a bare git repository so that
we don't need to create a git index file. The new parameter need a git
version >= 2.40 . If git version less than 2.40, it will fall back to
previous implementation.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: yp05327 <576951401@qq.com>

modules/git/languagestats/language_stats_test.go

@@ -0,0 +1,46 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build !gogit
+
+package languagestats
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/modules/git"
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRepository_GetLanguageStats(t *testing.T) {
+	setting.AppDataPath = t.TempDir()
+	repoPath := "../tests/repos/language_stats_repo"
+	gitRepo, err := git.OpenRepository(t.Context(), repoPath)
+	require.NoError(t, err)
+	defer gitRepo.Close()
+
+	stats, err := GetLanguageStats(gitRepo, "8fee858da5796dfb37704761701bb8e800ad9ef3")
+	require.NoError(t, err)
+
+	assert.Equal(t, map[string]int64{
+		"Python": 134,
+		"Java":   112,
+	}, stats)
+}
+
+func TestMergeLanguageStats(t *testing.T) {
+	assert.Equal(t, map[string]int64{
+		"PHP":    1,
+		"python": 10,
+		"JAVA":   700,
+	}, mergeLanguageStats(map[string]int64{
+		"PHP":    1,
+		"python": 10,
+		"Java":   100,
+		"java":   200,
+		"JAVA":   400,
+	}))
+}