Save netlify branch to site.branch, and use that in editor config, defaulting to site.github branch

This is necessary because `site.github.source.branch` refers to the
upstream repo's branch, not the fork. See #227.

Author: timwis

editor/index.html

@@ -21,7 +21,7 @@
       config: {
         backend: {
           repo: {{ site.github.repository_nwo | jsonify }},
-          branch: {{ site.github.source.branch | jsonify }}
+          branch: {{ site.branch | default: site.github.source.branch | jsonify }}
         },
         local_backend: {% if jekyll.environment == "production" %}false{% else %}true{% endif %},
         site_url: {{ "" | absolute_url | jsonify }}

netlify/plugins/netlify-jekyll-metadata/index.js

@@ -5,8 +5,9 @@ export const onPreBuild = async function () {
   const siteUrl = process.env.URL
   const repoUrl = new URL(process.env.REPOSITORY_URL || '')
   const repoNameWithOwner = repoUrl.pathname.replace(/^\/+/, '')
+  const branch = process.env.BRANCH
 
   const fileName = '_config_override.yml'
-  const fileContents = `repository: ${repoNameWithOwner}\nurl: ${siteUrl}\nbaseurl: ""`
+  const fileContents = `repository: ${repoNameWithOwner}\nurl: ${siteUrl}\nbranch: ${branch}\nbaseurl: ""`
   return fs.promises.writeFile(fileName, fileContents)
 }