Full-file syntax highlighting for diff pages (#33766)

Fix #33358, fix #21970

This adds a step in the `GitDiffForRender` that does syntax highlighting for the
entire file and then only references lines from that syntax highlighted
code. This allows things like multi-line comments to be syntax
highlighted correctly.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/gitdiff/gitdiff_test.go

@@ -17,27 +17,10 @@ import (
 	"code.gitea.io/gitea/modules/json"
 	"code.gitea.io/gitea/modules/setting"
 
-	dmp "github.com/sergi/go-diff/diffmatchpatch"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
 
-func TestDiffToHTML(t *testing.T) {
-	assert.Equal(t, "foo <span class=\"added-code\">bar</span> biz", diffToHTML(nil, []dmp.Diff{
-		{Type: dmp.DiffEqual, Text: "foo "},
-		{Type: dmp.DiffInsert, Text: "bar"},
-		{Type: dmp.DiffDelete, Text: " baz"},
-		{Type: dmp.DiffEqual, Text: " biz"},
-	}, DiffLineAdd))
-
-	assert.Equal(t, "foo <span class=\"removed-code\">bar</span> biz", diffToHTML(nil, []dmp.Diff{
-		{Type: dmp.DiffEqual, Text: "foo "},
-		{Type: dmp.DiffDelete, Text: "bar"},
-		{Type: dmp.DiffInsert, Text: " baz"},
-		{Type: dmp.DiffEqual, Text: " biz"},
-	}, DiffLineDel))
-}
-
 func TestParsePatch_skipTo(t *testing.T) {
 	type testcase struct {
 		name        string
@@ -621,7 +604,7 @@ func TestGetDiffRangeWithWhitespaceBehavior(t *testing.T) {
 
 	defer gitRepo.Close()
 	for _, behavior := range []git.TrustedCmdArgs{{"-w"}, {"--ignore-space-at-eol"}, {"-b"}, nil} {
-		diffs, err := GetDiff(t.Context(), gitRepo,
+		diffs, err := GetDiffForAPI(t.Context(), gitRepo,
 			&DiffOptions{
 				AfterCommitID:      "d8e0bbb45f200e67d9a784ce55bd90821af45ebd",
 				BeforeCommitID:     "72866af952e98d02a73003501836074b286a78f6",