refactor: git patch apply (#38637)

Merge duplicate code and add a unit test

There are already integration test cases in `TestEditor` ->
`WebGitCommitEmail` for these two endpoints.

services/repository/files/patch_test.go

@@ -0,0 +1,38 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package files
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	repo_model "gitea.dev/models/repo"
+	"gitea.dev/models/unittest"
+	user_model "gitea.dev/models/user"
+	"gitea.dev/modules/git"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestGitPatchPrepare(t *testing.T) {
+	unittest.PrepareTestEnv(t)
+	user2 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
+	repo1 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})
+
+	gitRepo, err := git.OpenRepository(repo1.CodeStorageRepo())
+	require.NoError(t, err)
+	defer gitRepo.Close()
+
+	opts := &ApplyDiffPatchOptions{}
+	tmpRepo, err := gitPatchPrepare(t.Context(), repo1, gitRepo, user2, opts)
+	require.NoError(t, err)
+	defer tmpRepo.Close()
+	// Temporary repository for patch should not be a bare repo because "--index" argument is used in some cases:
+	// * repo files might be written into current working tree when "applying the patch" then overwrite bare repo's git dir internal files.
+	// * see also "FIXME: GIT-DIR-ARGUMENT"
+	// While it is also questionable whether "--index" argument should really be used, need to figure out in the future.
+	_, err = os.Stat(filepath.Join(tmpRepo.basePath, ".git"))
+	require.NoError(t, err)
+}