Fix conflict highlighting

Conflicts used to take a `Repository` and pass that to
`Gitlab::Highlight.highlight`, which would call `#gitattribute` on the
repository. Now they use a `Gitlab::Git::Repository`, which didn't have that
method defined - but defining it on `Gitlab::Git::Repository` does make it
available on `Repository` through `method_missing`, so we can do that and both
cases will work.

app/models/repository.rb

@@ -994,10 +994,6 @@ class Repository
     raw_repository.ls_files(actual_ref)
   end
-  def gitattribute(path, name)
-    raw_repository.attributes(path)[name]
-  end
-
   def copy_gitattributes(ref)
     actual_ref = ref || root_ref
     begin

lib/gitlab/git/repository.rb

@@ -984,6 +984,10 @@ module Gitlab
         @attributes.attributes(path)
       end
+      def gitattribute(path, name)
+        attributes(path)[name]
+      end
+
       def languages(ref = nil)
         Gitlab::GitalyClient.migrate(:commit_languages) do |is_enabled|
           if is_enabled

spec/lib/gitlab/conflict/file_spec.rb

@@ -84,6 +84,13 @@ describe Gitlab::Conflict::File do
         expect(line.text).to eq(html_to_text(line.rich_text))
       end
     end
+
+    # This spec will break if Rouge's highlighting changes, but we need to
+    # ensure that the lines are actually highlighted.
+    it 'highlights the lines correctly' do
+      expect(conflict_file.lines.first.rich_text)
+        .to eq("<span id=\"LC1\" class=\"line\" lang=\"ruby\"><span class=\"k\">module</span> <span class=\"nn\">Gitlab</span></span>\n")
+    end
   end
   describe '#sections' do