Allow cap -T to work when no Capfile is present

Previously, the `cap -T` command assumed the presence of the
`load:defaults` task. This task is only present if a proper Capfile is
loaded, and so `cap -T` wouldn't work in a new, not yet "capified"
project.

This commit fixes the bug by testing that `load:defaults` is defined
before calling it.

CHANGELOG.md

@@ -13,6 +13,7 @@ gem "capistrano", :github => "capistrano/capistrano"
 https://github.com/capistrano/capistrano/compare/v3.7.2...HEAD
 
 * Your contribution here!
+* [#1867](https://github.com/capistrano/capistrano/pull/1867): Allow `cap -T` to run without Capfile present - [@mattbrictson](https://github.com/mattbrictson)
 
 ## `3.8.0` (2017-03-10)
 

features/installation.feature

@@ -3,6 +3,11 @@ Feature: Installation
   Background:
     Given a test app without any configuration
 
+  Scenario: The "install" task documentation can be viewed
+    When I run "cap -T"
+    Then the task is successful
+    And contains "cap install" in the output
+
   Scenario: With default stages
     When I run "cap install"
     Then the deploy.rb file is created

lib/capistrano/application.rb

@@ -96,7 +96,7 @@ module Capistrano
     end
 
     def load_imports
-      if options.show_tasks
+      if options.show_tasks && Rake::Task.task_defined?("load:defaults")
         invoke "load:defaults"
         set(:stage, "")
         Dir[deploy_config_path].each { |f| add_import f }