Set Asciidoctor outfilesuffix to default .adoc

Without setting outfilesuffix Asciidoctor defaults to .html. When .html is used,
inter-document cross references do not work in Gitlab's repository / file renderer.
See more information from
http://asciidoctor.org/docs/user-manual/#navigating-between-source-files
and
http://asciidoctor.org/docs/user-manual/#inter-document-cross-references

changelogs/unreleased/asciidoc_inter_document_cross_references.yml

+---
+title: Asciidoc now support inter-document cross references between files in repository
+merge_request: 17125
+author: Turo Soisenniemi
+type: changed

lib/gitlab/asciidoc.rb

@@ -8,7 +8,8 @@ module Gitlab
   module Asciidoc
     DEFAULT_ADOC_ATTRS = [
       'showtitle', 'idprefix=user-content-', 'idseparator=-', 'env=gitlab',
-      'env-gitlab', 'source-highlighter=html-pipeline', 'icons=font'
+      'env-gitlab', 'source-highlighter=html-pipeline', 'icons=font',
+      'outfilesuffix=.adoc'
     ].freeze
 
     # Public: Converts the provided Asciidoc markup into HTML.

spec/lib/gitlab/asciidoc_spec.rb

@@ -95,6 +95,14 @@ module Gitlab
           expect(render(input, context)).to include('<p><code data-math-style="inline" class="code math js-render-math">2+2</code> is 4</p>')
         end
       end
+
+      context 'outfilesuffix' do
+        it 'defaults to adoc' do
+          output = render("Inter-document reference <<README.adoc#>>", context)
+
+          expect(output).to include("a href=\"README.adoc\"")
+        end
+      end
     end
 
     def render(*args)