perf(gitdiff): optimize inline diff highlighting using cache (#38706)

This pull request optimizes code diff highlighting in the PR Files
Changed view, which is a major CPU bottleneck and hot path in
production.

Previously, Gitea executed the expensive `DiffMatchPatch` algorithm
twice for every matching deleted/added line pair (once for the deleted
line, and once for the added line). We now run the diff algorithm once
and cache the resulting `DiffInline` on the `DiffLine` struct itself,
allowing the second line to render via a 0-cost cache lookup.

---------

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

services/gitdiff/gitdiff_test.go

@@ -18,6 +18,7 @@ import (
 	"gitea.dev/modules/git/gitcmd"
 	"gitea.dev/modules/json"
 	"gitea.dev/modules/setting"
+	"gitea.dev/modules/translation"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -1109,6 +1110,15 @@ func TestDiffLine_GetExpandDirection(t *testing.T) {
 	}
 }
 
+func TestDiffSection_GetComputedInlineDiffFor(t *testing.T) {
+	t.Run("Section", func(t *testing.T) {
+		diffLine := &DiffLine{Type: DiffLineSection, Content: "@@ -1,3 +1,3 @@ func \u202ename() <b>"}
+		diffInline := (&DiffSection{}).GetComputedInlineDiffFor(diffLine, translation.MockLocale{})
+		assert.True(t, diffInline.EscapeStatus.Escaped)
+		assert.Equal(t, `@@ -1,3 +1,3 @@ func <span class="escaped-code-point" data-escaped="[U+202E]"><span class="char">`+"\u202e"+`</span></span>name() &lt;b&gt;`, string(diffInline.Content))
+	})
+}
+
 func TestHighlightCodeLines(t *testing.T) {
 	t.Run("CharsetDetecting", func(t *testing.T) {
 		diffFile := &DiffFile{