net/http: merge IsDir checks in fs.go's serveFile function

Fixes #33385

Change-Id: I497ccd868d408a9c5648c72aa5ce41221368daf4
GitHub-Last-Rev: 3bf483808e10a13c522df02c2fc55fe278044680
GitHub-Pull-Request: golang/go#33423
Reviewed-on: https://go-review.googlesource.com/c/go/+/188677Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/fs.go

@@ -582,17 +582,15 @@ func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirec
 		}
 	}
 
-	// redirect if the directory name doesn't end in a slash
 	if d.IsDir() {
 		url := r.URL.Path
+		// redirect if the directory name doesn't end in a slash
 		if url == "" || url[len(url)-1] != '/' {
 			localRedirect(w, r, path.Base(url)+"/")
 			return
 		}
-	}
 
-	// use contents of index.html for directory, if present
-	if d.IsDir() {
+		// use contents of index.html for directory, if present
 		index := strings.TrimSuffix(name, "/") + indexPage
 		ff, err := fs.Open(index)
 		if err == nil {