fix($hmr): Prepend relative publicPath to new filename path.
fix($hmr): Prepend relative publicPath to new filename path.

Previous code dropped publicPath on the floor when it was relative, which would have broken certain
corner cases.

brightbytes/extract-css-chunks-webpack-plugin,faceyspacey/extract-css-chunks-webpack-plugin,brightbytes/extract-css-chunks-webpack-plugin,faceyspacey/extract-css-chunks-webpack-plugin