Make it possible to specifiy only: changes keywords

lib/gitlab/ci/config/entry/policy.rb

@@ -25,17 +25,19 @@ module Gitlab
             include Entry::Validatable
             include Entry::Attributable
 
-            attributes :refs, :kubernetes, :variables
+            ALLOWED_KEYS = %i[refs kubernetes variables changes]
+            attributes :refs, :kubernetes, :variables, :changes
 
             validations do
               validates :config, presence: true
-              validates :config, allowed_keys: %i[refs kubernetes variables]
+              validates :config, allowed_keys: ALLOWED_KEYS
               validate :variables_expressions_syntax
 
               with_options allow_nil: true do
                 validates :refs, array_of_strings_or_regexps: true
                 validates :kubernetes, allowed_values: %w[active]
                 validates :variables, array_of_strings: true
+                validates :changes, array_of_strings: true
               end
 
               def variables_expressions_syntax

spec/lib/gitlab/ci/config/entry/policy_spec.rb

-require 'spec_helper'
+require 'fast_spec_helper'
+require_dependency 'active_model'
 
 describe Gitlab::Ci::Config::Entry::Policy do
   let(:entry) { described_class.new(config) }
@@ -124,6 +125,23 @@ describe Gitlab::Ci::Config::Entry::Policy do
       end
     end
 
+    context 'when specifying a valid changes policy' do
+      let(:config) { { changes: %w[some/* paths/**/*.rb] } }
+
+      it 'is a correct configuraton' do
+        expect(entry).to be_valid
+        expect(entry.value).to eq(config)
+      end
+    end
+
+    context 'when changes policy is invalid' do
+      let(:config) { { changes: [1, 2] } }
+
+      it 'returns errors' do
+        expect(entry.errors).to include /changes should be an array of strings/
+      end
+    end
+
     context 'when specifying unknown policy' do
       let(:config) { { refs: ['master'], invalid: :something } }