You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: diff_test.go
    path of file: ./repos/gitea/services/repository/files
    
    the code of the file until where you have to start completion: // Copyright 2019 The Gitea Authors. All rights reserved.
// SPDX-License-Identifier: MIT

package files

import (
	"testing"

	repo_model "code.gitea.io/gitea/models/repo"
	"code.gitea.io/gitea/models/unittest"
	"code.gitea.io/gitea/modules/json"
	"code.gitea.io/gitea/services/contexttest"
	"code.gitea.io/gitea/services/gitdiff"

	"github.com/stretchr/testify/assert"
)

func TestGetDiffPreview(t *testing.T) {
	unittest.PrepareTestEnv(t)
	ctx, _ := contexttest.MockContext(t, "user2/repo1")
	ctx.SetParams(":id", "1")
	contexttest.LoadRepo(t, ctx, 1)
	contexttest.LoadRepoCommit(t, ctx)
	contexttest.LoadUser(t, ctx, 2)
	contexttest.LoadGitRepo(t, ctx)
	defer ctx.Repo.GitRepo.Close()

	branch := ctx.Repo.Repository.DefaultBranch
	treePath := "READM
    
func TestGetDiffPreviewErrors(t *testing.T) {
	unittest.PrepareTestEnv(t)
	ctx, _ := contexttest.MockContext(t, "user2/repo1")
	ctx.SetParams(":id", "1")
	contexttest.LoadRepo(t, ctx, 1)
	contexttest.LoadRepoCommit(t, ctx)
	contexttest.LoadUser(t, ctx, 2)
	contexttest.LoadGitRepo(t, ctx)
	defer ctx.Repo.GitRepo.Close()

	branch := ctx.Repo.Repository.DefaultBranch
	treePath := "README.md"
	content := "# repo1\n\nDescription for repo1\nthis is a new line"

	t.Run("empty repo", func(t *testing.T) {
		diff, err := GetDiffPreview(ctx, &repo_model.Repository{}, branch, treePath, content)
		assert.Nil(t, diff)
		assert.EqualError(t, err, "repository does not exist [id: 0, uid: 0, owner_name: , name: ]")
	})

	t.Run("bad branch", func(t *testing.T) {
		badBranch := "bad_branch"
		diff, err := GetDiffPreview(ctx, ctx.Repo.Repository, badBranch, treePath, content)
		assert.Nil(t, diff)
		assert.EqualError(t, err, "branch does not exist [name: "+badBranch+"]")
	})

	t.Run("empty treePath", func(t *testing.T) {
		diff, err := GetDiffPreview(ctx, ctx.Repo.Repository, branch, "", content)
		assert.Nil(t, diff)
		assert.EqualError(t, err, "path is invalid [path: ]")
	})
}