Introduce spec/fast_spec_helper.rb to run spec files that don't rely on the whole Rails env

Signed-off-by: Rémy Coutable <remy@rymai.me>

config/initializers/2_app.rb

+require_dependency 'gitlab/popen'
+
 module Gitlab
   def self.config
     Settings

lib/gitlab/git.rb

+require_dependency 'gitlab/encoding_helper'
+
 module Gitlab
   module Git
     # The ID of empty tree.

rubocop/spec_helpers.rb

 module RuboCop
   module SpecHelpers
-    SPEC_HELPERS = %w[spec_helper.rb rails_helper.rb].freeze
+    SPEC_HELPERS = %w[fast_spec_helper.rb rails_helper.rb spec_helper.rb].freeze
 
     # Returns true if the given node originated from the spec directory.
     def in_spec?(node)

spec/fast_spec_helper.rb

+require 'bundler/setup'
+
+require 'settingslogic'
+
+ENV["RAILS_ENV"] = 'test'
+ENV["IN_MEMORY_APPLICATION_SETTINGS"] = 'true'
+
+unless Kernel.respond_to?(:require_dependency)
+  module Kernel
+    alias_method :require_dependency, :require
+  end
+end
+
+unless defined?(Rails)
+  module Rails
+    def self.root
+      Pathname.new(File.expand_path(''))
+    end
+
+    # Copied from https://github.com/rails/rails/blob/v4.2.10/railties/lib/rails.rb#L59-L61
+    def self.env
+      @_env ||= ActiveSupport::StringInquirer.new(ENV["RAILS_ENV"] || ENV["RACK_ENV"] || "development")
+    end
+  end
+end
+
+# Settings is used in config/initializers/2_app.rb
+class Settings < Settingslogic
+  source Rails.root.join('config/gitlab.yml')
+  namespace Rails.env
+end
+
+# Defines Gitlab and Gitlab.config
+unless defined?(Gitlab) && Gitlab.respond_to?(:config)
+  require_relative '../config/initializers/2_app'
+end
+
+require_relative 'support/rspec'

spec/lib/gitlab_spec.rb

-require 'rails_helper'
+require 'fast_spec_helper'
+
+require_dependency 'gitlab'
 
 describe Gitlab do
   describe '.com?' do