Add auxiliary blob viewer for CHANGELOG

app/models/blob.rb

@@ -41,7 +41,8 @@ class Blob < SimpleDelegator
     BlobViewer::RouteMap,
 
     BlobViewer::License,
-    BlobViewer::Contributing
+    BlobViewer::Contributing,
+    BlobViewer::Changelog
   ].freeze
 
   attr_reader :project

app/models/blob_viewer/base.rb

@@ -11,6 +11,8 @@ module BlobViewer
     attr_reader :blob
     attr_accessor :override_max_size
 
+    delegate :project, to: :blob
+
     def initialize(blob)
       @blob = blob
     end

app/models/blob_viewer/changelog.rb

+module BlobViewer
+  class Changelog < Base
+    include Auxiliary
+    include Static
+
+    self.partial_name = 'changelog'
+    self.file_types = %i(changelog)
+    self.binary = false
+
+    def render_error
+      return if project.repository.tag_count > 0
+
+      :no_tags
+    end
+  end
+end

app/models/blob_viewer/license.rb

@@ -8,7 +8,7 @@ module BlobViewer
     self.binary = false
 
     def license
-      blob.project.repository.license
+      project.repository.license
     end
 
     def render_error

app/views/projects/blob/viewers/_changelog.html.haml

+= icon('history fw')
+= succeed '.' do
+  To find the state of this project's repository at the time of any of these versions, check out
+  = link_to "the tags", namespace_project_tags_path(viewer.project.namespace, viewer.project)

spec/models/blob_viewer/changelog_spec.rb

+require 'spec_helper'
+
+describe BlobViewer::Changelog, model: true do
+  include FakeBlobHelpers
+
+  let(:project) { create(:project, :repository) }
+  let(:blob) { fake_blob(path: 'CHANGELOG') }
+  subject { described_class.new(blob) }
+
+  describe '#render_error' do
+    context 'when there are no tags' do
+      before do
+        allow(project.repository).to receive(:tag_count).and_return(0)
+      end
+
+      it 'returns :no_tags' do
+        expect(subject.render_error).to eq(:no_tags)
+      end
+    end
+
+    context 'when there are tags' do
+      it 'returns nil' do
+        expect(subject.render_error).to be_nil
+      end
+    end
+  end
+end