Browser extension errors with Error Cannot read property 'title' of null when PR contains suggested change

Created by: felixfbecker

Visit a PR with a suggested change, e.g. https://github.com/sourcegraph/sourcegraph/pull/2258/files#r261230964

getDeltaFileName() will be called with the suggestion (wrongly assuming it is a code view) and will fail to find the file name on it.

This is with the browser extension on master.