fix: rebase open authoring branches (#2975)

* fix: create open authoring branches from default origin branch

* refactor: pass repoURL to getBranch

* fix(backend-github): rebase open authoring branch,support multiple files

cypress/integration/github/open_authoring.js

@@ -64,7 +64,7 @@ export default function({ use_graphql }) {
   });
 
   it('can update a draft entry on fork', () => {
-    login(taskResult.data.user);
+    login(taskResult.data.forkUser);
     createPostAndExit(entry1);
     updateExistingPostAndExit(entry1, entry2);
   });