Remove legacy git code (ver < 2.0), fine tune markup tests (#19930)

* clean git support for ver < 2.0

* fine tune tests for markup (which requires git module)

* remove unnecessary comments

* try to fix tests

* try test again

* use const for GitVersionRequired instead of var

* try to fix integration test

* Refactor CheckAttributeReader to make a *git.Repository version

* update document for commit signing with Gitea's internal gitconfig

* update document for commit signing with Gitea's internal gitconfig

Co-authored-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/git/repo_language_stats_gogit.go

@@ -8,12 +8,10 @@ package git
 
 import (
 	"bytes"
-	"context"
 	"io"
 	"strings"
 
 	"code.gitea.io/gitea/modules/analyze"
-	"code.gitea.io/gitea/modules/log"
 
 	"github.com/go-enry/go-enry/v2"
 	"github.com/go-git/go-git/v5"
@@ -43,33 +41,8 @@ func (repo *Repository) GetLanguageStats(commitID string) (map[string]int64, err
 		return nil, err
 	}
 
-	var checker *CheckAttributeReader
-
-	if CheckGitVersionAtLeast("1.7.8") == nil {
-		indexFilename, workTree, deleteTemporaryFile, err := repo.ReadTreeToTemporaryIndex(commitID)
-		if err == nil {
-			defer deleteTemporaryFile()
-			checker = &CheckAttributeReader{
-				Attributes: []string{"linguist-vendored", "linguist-generated", "linguist-language", "gitlab-language"},
-				Repo:       repo,
-				IndexFile:  indexFilename,
-				WorkTree:   workTree,
-			}
-			ctx, cancel := context.WithCancel(DefaultContext)
-			if err := checker.Init(ctx); err != nil {
-				log.Error("Unable to open checker for %s. Error: %v", commitID, err)
-			} else {
-				go func() {
-					err = checker.Run()
-					if err != nil {
-						log.Error("Unable to open checker for %s. Error: %v", commitID, err)
-						cancel()
-					}
-				}()
-			}
-			defer cancel()
-		}
-	}
+	checker, deferable := repo.CheckAttributeReader(commitID)
+	defer deferable()
 
 	sizes := make(map[string]int64)
 	err = tree.Files().ForEach(func(f *object.File) error {