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>

routers/web/devtest/devtest.go

@@ -288,7 +288,7 @@ func prepareMockDataUnicodeEscape(ctx *context.Context) {
 	lineEscapeStatus := make([]*charset.EscapeStatus, len(highlightLines))
 	for i, hl := range highlightLines {
 		lineEscapeStatus[i], hl.FormattedContent = charset.EscapeControlHTML(hl.FormattedContent, ctx.Locale)
-		escapeStatus = escapeStatus.Or(lineEscapeStatus[i])
+		escapeStatus.Combine(lineEscapeStatus[i])
 	}
 	ctx.Data["HighlightLines"] = highlightLines
 	ctx.Data["EscapeStatus"] = escapeStatus