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>

routers/web/repo/blame.go

@@ -97,7 +97,7 @@ func RefBlame(ctx *context.Context) {
 		return
 	}
 
-	ctx.Data["NumLines"], err = blob.GetBlobLineCount()
+	ctx.Data["NumLines"], err = blob.GetBlobLineCount(nil)
 	if err != nil {
 		ctx.NotFound(err)
 		return