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/repo/view_file.go

@@ -127,7 +127,7 @@ func handleFileViewRenderSource(ctx *context.Context, attrs *attribute.Attribute
 	statuses := make([]*charset.EscapeStatus, len(fileContent))
 	for i, line := range fileContent {
 		statuses[i], fileContent[i] = charset.EscapeControlHTML(line, ctx.Locale)
-		status = status.Or(statuses[i])
+		status.Combine(statuses[i])
 	}
 	ctx.Data["EscapeStatus"] = status
 	ctx.Data["FileContent"] = fileContent