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

packages/netlify-cms-backend-git-gateway/src/GitHubAPI.js

@@ -12,7 +12,7 @@ export default class API extends GithubAPI {
   }
 
   hasWriteAccess() {
-    return this.getBranch()
+    return this.getDefaultBranch()
       .then(() => true)
       .catch(error => {
         if (error.status === 401) {