fix watermark_branch

Author: osfameron

lib/preview.js

@@ -18,6 +18,9 @@ module.exports.register = function () {
     const args = {
         repo: process.env.PREVIEW_REPO,
         branch: process.env.PREVIEW_BRANCH || 'HEAD',
+        // watermark branch allows us to masquerade as a different branch
+        // this is useful for previewing a PR against the main branch e.g.
+        // so that the watermark lines don't trigger in the diff.
         watermark_branch: process.env.PREVIEW_WATERMARK_BRANCH,
         config: process.env.PREVIEW_CONFIG || process.env.PREVIEW_BRANCH || 'HEAD',
         remote: process.env.PREVIEW_REMOTE ? true : false,
@@ -136,7 +139,7 @@ module.exports.register = function () {
     playbook.content.sources = mappedSources
 
     const date = new Date().toISOString().split('T')[0]
-    const watermark_branch = args.watermark || args.branch
+    const watermark_branch = args.watermark_branch || args.branch
     playbook.asciidoc.attributes['page-watermark'] =
         `${date} ${args.repo} ${watermark_branch}`