Improve method that tells if build is retryable

This method now should return false if build is not completed. If build
is running then it is not retryable, therefore `Ci::Build#retryable?`
returned wrong value. This commit changes this behavior

app/models/ci/build.rb

@@ -90,7 +90,7 @@ module Ci
     end
 
     def retryable?
-      project.builds_enabled? && commands.present?
+      project.builds_enabled? && commands.present? && complete?
     end
 
     def retried?

app/views/projects/builds/_sidebar.html.haml

@@ -40,7 +40,7 @@
   .block{ class: ("block-first" if !@build.coverage && !(can?(current_user, :read_build, @project) && (@build.artifacts? || @build.artifacts_expired?))) }
     .title
       Build details
-      - if @build.retryable? && !@build.active?
+      - if @build.retryable?
         = link_to "Retry", retry_namespace_project_build_path(@project.namespace, @project, @build), class: 'pull-right', method: :post
     - if @build.merge_request
       %p.build-detail-row

spec/models/build_spec.rb

@@ -669,4 +669,22 @@ describe Ci::Build, models: true do
       expect(build.commit).to eq project.commit
     end
   end
+
+  describe '#retryable?' do
+    context 'when build is running' do
+      before { build.run! }
+
+      it 'should return false' do
+        expect(build.retryable?).to be false
+      end
+    end
+
+    context 'when build is finished' do
+      before { build.success! }
+
+      it 'should return true' do
+        expect(build.retryable?).to be true
+      end
+    end
+  end
 end