Fix import issue to unblock node-fetch update (#189)

* switch import to require

Author: tim-finnigan

src/process-diff.ts

@@ -1,5 +1,6 @@
 import * as parse from 'parse-diff';
-import fetch, {Headers} from 'node-fetch';
+const fetch = require('node-fetch');
+import {Headers} from 'node-fetch';
 
 export async function processDiffUrl(
   htmlUrl: string,