Speed up used languages calculation on charts page

We removed calls from our code to Rugged::Repository#fetch_attributes:
<https://gitlab.com/gitlab-org/gitlab_git/commit/340e111e040ae847b614d35b4d3173ec48329015>

However, we didn't remove calls from within Linguist. This method is only called
when calculating the languages for a repository on the Charts page:
<https://github.com/github/linguist/blob/v4.7.0/lib/linguist/lazy_blob.rb#L33-L36>

We can safely use our own Gitlab::Git::Attributes here. On staging, for the
GitLab CE repo, this makes the calculation take about a third of the time:

    # Before
    Benchmark.realtime do
      Linguist::Repository.new(repository.rugged,
                               repository.rugged.head.target_id).languages
    end
    #=> 23.67193900188431

    # After
    Benchmark.realtime do
      Linguist::Repository.new(repository.rugged,
                               repository.rugged.head.target_id).languages
    end
    #=> 8.945212290156633

changelogs/unreleased/speed-up-graphs.yml

+---
+title: Speed up used languages calculation on charts page
+merge_request:
+author:

config/initializers/rugged_use_gitlab_git_attributes.rb

+# We don't want to ever call Rugged::Repository#fetch_attributes, because it has
+# a lot of I/O overhead:
+# <https://gitlab.com/gitlab-org/gitlab_git/commit/340e111e040ae847b614d35b4d3173ec48329015>
+#
+# While we don't do this from within the GitLab source itself, the Linguist gem
+# has a dependency on Rugged and uses the gitattributes file when calculating
+# repository-wide language statistics:
+# <https://github.com/github/linguist/blob/v4.7.0/lib/linguist/lazy_blob.rb#L33-L36>
+#
+# The options passed by Linguist are those assumed by Gitlab::Git::Attributes
+# anyway, and there is no great efficiency gain from just fetching the listed
+# attributes with our implementation, so we ignore the additional arguments.
+#
+module Rugged
+  class Repository
+    module UseGitlabGitAttributes
+      def fetch_attributes(name, *)
+        @attributes ||= Gitlab::Git::Attributes.new(path)
+        @attributes.attributes(name)
+      end
+    end
+
+    prepend UseGitlabGitAttributes
+  end
+end