Redirect a directory request with a relative Location (#2630)

* Redirect a directory request with a relative Location

The trailing-slash redirect used the request path as-is, so a path
starting with "//" produced a Location that clients read as a
network-path reference to another host. Name only the last segment,
percent-encoded and prefixed with "./", so the target always resolves
against the request URL, whatever the mount point or any prefix a
proxy put in front of it.

* Shorten the relative redirect comment

httplib.h

@@ -13869,7 +13869,11 @@ inline bool Server::handle_file_request(Request &req, Response &res) {
         detail::FileStat stat(path);
 
         if (stat.is_dir()) {
-          res.set_redirect(req.path + "/", StatusCode::MovedPermanently_301);
+          // Relative, so "//dir" cannot name another host; "./" keeps a ':'
+          // in the name from reading as a scheme.
+          auto name = req.path.substr(req.path.rfind('/') + 1);
+          res.set_redirect("./" + encode_path_component(name) + "/",
+                           StatusCode::MovedPermanently_301);
           return true;
         }