Speed up commit loads by disabling BatchLoader replace_methods

We've seen a significant performance penalty when using
`BatchLoader#__replace_with!`. This defines methods on the batch loader
that proxy to the 'real' object using send. The alternative is
`method_missing`, which is slower.  However, we've noticed that
`method_missing` can be faster if:

1. The objects being loaded have a large interface.
2. We don't call too many methods on the loaded object.

In production, we've seen the rendering times of the merge request
widget increase as a result of loading commit data. BatchLoader attempts
to call replace_methods on the lazy object, but this has a significant
performance penalty. Disabling this mode
(exAspArk/batch-loader#45) appears to cut load
times by about 50% for MergeRequestsController#show.

Relates to https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/6941

changelogs/unreleased/sh-speed-up-commit-loading.yml

@@ -0,0 +1,5 @@
+---
+title: Speed up commit loads by disabling BatchLoader replace_methods
+merge_request:
+author:
+type: performance

spec/models/commit_spec.rb

@@ -44,6 +44,14 @@
           expect(commit.id).to eq(oids[i])
         end
       end
+
+      it 'does not attempt to replace methods via BatchLoader' do
+        subject.each do |commit|
+          expect(commit).to receive(:method_missing).and_call_original
+
+          commit.id
+        end
+      end
     end
 
     context 'when not found' do