Add and use Ci::Build#retryable? method.

app/controllers/projects/builds_controller.rb

@@ -40,7 +40,7 @@ class Projects::BuildsController < Projects::ApplicationController
   end
   def retry
-    if @build.commands.blank?
+    unless @build.retryable?
       return page_404
     end

app/models/ci/build.rb

@@ -106,6 +106,9 @@ module Ci
       failed? && allow_failure?
     end
+    def retryable?
+      commands.present?
+    end
     def trace_html
       html = Ci::Ansi2html::convert(trace) if trace.present?
       html || ''
@@ -222,7 +225,7 @@ module Ci
     end
     def retry_url
-      if commands.present?
+      if retryable?
         Gitlab::Application.routes.url_helpers.
           retry_namespace_project_build_path(gl_project.namespace, gl_project, self)
       end