Enable Gitaly ref name caching for discussions.json

This eliminates many potential duplicate FindCommit RPCs for the same
ref, which often occurs in the RelativeLinkFilter#current_commit call.
On the GitLab 12.0 release post, for example, this would save close to
400 RPC calls.

app/controllers/projects/merge_requests_controller.rb

@@ -16,7 +16,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
   before_action :authenticate_user!, only: [:assign_related_issues]
   before_action :check_user_can_push_to_source_branch!, only: [:rebase]
 
-  around_action :allow_gitaly_ref_name_caching, only: [:index, :show]
+  around_action :allow_gitaly_ref_name_caching, only: [:index, :show, :discussions]
 
   def index
     @merge_requests = @issuables

changelogs/unreleased/sh-enable-ref-name-caching-discussions.yml

+---
+title: Enable Gitaly ref name caching for discussions.json
+merge_request: 29951
+author:
+type: performance

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -1001,6 +1001,8 @@ describe Projects::MergeRequestsController do
       before do
         project.add_developer(user)
         sign_in(user)
+
+        expect(::Gitlab::GitalyClient).to receive(:allow_ref_name_caching).and_call_original
       end
 
       it 'returns 200' do