Support repo code search without setting up an indexer (#29998)

By using git's ability, end users (especially small instance users) do
not need to enable the indexer, they could also benefit from the code
searching feature.

Fix #29996

![image](11b7e458-88a4-480d-b4d7-72ee59406dd1)

![image](0fe777d5-c95c-4288-a818-0427680805b6)

---------

Co-authored-by: silverwind <me@silverwind.io>

modules/git/grep_test.go

@@ -0,0 +1,41 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+import (
+	"context"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGrepSearch(t *testing.T) {
+	repo, err := openRepositoryWithDefaultContext(filepath.Join(testReposDir, "language_stats_repo"))
+	assert.NoError(t, err)
+	defer repo.Close()
+
+	res, err := GrepSearch(context.Background(), repo, "void", GrepOptions{})
+	assert.NoError(t, err)
+	assert.Equal(t, []*GrepResult{
+		{
+			Filename:    "java-hello/main.java",
+			LineNumbers: []int{3},
+			LineCodes:   []string{" public static void main(String[] args)"},
+		},
+		{
+			Filename:    "main.vendor.java",
+			LineNumbers: []int{3},
+			LineCodes:   []string{" public static void main(String[] args)"},
+		},
+	}, res)
+
+	res, err = GrepSearch(context.Background(), repo, "no-such-content", GrepOptions{})
+	assert.NoError(t, err)
+	assert.Len(t, res, 0)
+
+	res, err = GrepSearch(context.Background(), &Repository{Path: "no-such-git-repo"}, "no-such-content", GrepOptions{})
+	assert.Error(t, err)
+	assert.Len(t, res, 0)
+}