Ensure to fetch before checking out a new Git ref

build-docs.rb

@@ -29,6 +29,7 @@ config['projects'].each do |project_name, project_config|
     repo = Git.clone(project_config['git'], clone_target)
   else
     repo = Git.open(clone_target)
+    repo.fetch()
   end
 
   puts "Checkout ref '#{project_config['ref']}'"
@@ -63,4 +64,4 @@ end
 
 File.write('mkdocs.yml', mkdocs.to_yaml)
 
-%x( mkdocs build )
+%x( mkdocs build )