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/merge_tree_test.go

@@ -0,0 +1,82 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitrepo
+
+import (
+	"path/filepath"
+	"testing"
+
+	"code.gitea.io/gitea/modules/git/gitcmd"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func prepareRepoDirRenameConflict(t *testing.T) string {
+	repoDir := filepath.Join(t.TempDir(), "repo-dir-rename-conflict.git")
+	require.NoError(t, gitcmd.NewCommand("init", "--bare").AddDynamicArguments(repoDir).Run(t.Context()))
+	stdin := `blob
+mark :1
+data 2
+b
+
+blob
+mark :2
+data 2
+c
+
+reset refs/heads/master
+commit refs/heads/master
+mark :3
+author test <test@example.com> 1769202331 -0800
+committer test <test@example.com> 1769202331 -0800
+data 2
+O
+M 100644 :1 z/b
+M 100644 :2 z/c
+
+commit refs/heads/split
+mark :4
+author test <test@example.com> 1769202336 -0800
+committer test <test@example.com> 1769202336 -0800
+data 2
+A
+from :3
+M 100644 :2 w/c
+M 100644 :1 y/b
+D z/b
+D z/c
+
+blob
+mark :5
+data 2
+d
+
+commit refs/heads/add
+mark :6
+author test <test@example.com> 1769202342 -0800
+committer test <test@example.com> 1769202342 -0800
+data 2
+B
+from :3
+M 100644 :5 z/d
+`
+	require.NoError(t, gitcmd.NewCommand("fast-import").WithDir(repoDir).WithStdinBytes([]byte(stdin)).Run(t.Context()))
+	return repoDir
+}
+
+func TestMergeTreeDirectoryRenameConflictWithoutFiles(t *testing.T) {
+	repoDir := prepareRepoDirRenameConflict(t)
+	require.DirExists(t, repoDir)
+	repo := &mockRepository{path: repoDir}
+
+	mergeBase, err := MergeBase(t.Context(), repo, "add", "split")
+	require.NoError(t, err)
+
+	treeID, conflicted, conflictedFiles, err := MergeTree(t.Context(), repo, "add", "split", mergeBase)
+	require.NoError(t, err)
+	assert.True(t, conflicted)
+	assert.Empty(t, conflictedFiles)
+	assert.Equal(t, "5e3dd4cfc5b11e278a35b2daa83b7274175e3ab1", treeID)
+}