Make WikiPage comparable according to StaticModel interface

* Add WikiPage#id method returning associated SHA for wiki page commit

app/models/wiki_page.rb

@@ -207,6 +207,10 @@ class WikiPage
     'projects/wikis/wiki_page'
   end
+  def id
+    page.version.to_s
+  end
+
   private
   def set_attributes

spec/models/wiki_page_spec.rb

@@ -318,6 +318,19 @@ describe WikiPage, models: true do
     end
   end
+  describe '#==' do
+    let(:original_wiki_page) { create(:wiki_page) }
+
+    it 'returns true for identical wiki page' do
+      expect(original_wiki_page).to eq(original_wiki_page)
+    end
+
+    it 'returns false for updated wiki page' do
+      updated_wiki_page = original_wiki_page.update("Updated content")
+      expect(original_wiki_page).not_to eq(updated_wiki_page)
+    end
+  end
+
   private
   def remove_temp_repo(path)