Use merge tree to detect conflicts when possible (#36400)

In Git 2.38, the `merge-tree` command introduced the `--write-tree`
option, which works directly on bare repositories. In Git 2.40, a new parameter `--merge-base` introduced so we require Git 2.40 to use the merge tree feature.

This option produces the merged tree object ID, allowing us to perform
diffs between commits without creating a temporary repository. By
avoiding the overhead of setting up and tearing down temporary repos,
this approach delivers a notable performance improvement.

It also fixes a possible situation that conflict files might be empty
but it's a conflict status according to
https://git-scm.com/docs/git-merge-tree#_mistakes_to_avoid

Replace #35542

---------

Signed-off-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/gitrepo/main_test.go

@@ -4,29 +4,21 @@
 package gitrepo
 
 import (
-	"os"
 	"path/filepath"
 	"testing"
 
-	"code.gitea.io/gitea/modules/log"
-	"code.gitea.io/gitea/modules/setting"
-	"code.gitea.io/gitea/modules/tempdir"
+	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/test"
 )
 
 func TestMain(m *testing.M) {
-	gitHomePath, cleanup, err := tempdir.OsTempDir("gitea-test").MkdirTempRandom("git-home")
-	if err != nil {
-		log.Fatal("Unable to create temp dir: %v", err)
-	}
-	defer cleanup()
-
 	// resolve repository path relative to the test directory
 	testRootDir := test.SetupGiteaRoot()
 	repoPath = func(repo Repository) string {
-		return filepath.Join(testRootDir, "/modules/git/tests/repos", repo.RelativePath())
+		if filepath.IsAbs(repo.RelativePath()) {
+			return repo.RelativePath() // for testing purpose only
+		}
+		return filepath.Join(testRootDir, "modules/git/tests/repos", repo.RelativePath())
 	}
-
-	setting.Git.HomePath = gitHomePath
-	os.Exit(m.Run())
+	git.RunGitTests(m)
 }