Allow Gitaly ref name caching for issue discussions

Issue discussions can link to repository files as relative links. If
several comments do so, it means that we will fetch the default branch's
ref from Gitaly once per comment, when we don't expect this to change
during the request. (This caching is already enabled for an MR's
discussions.)

app/controllers/projects/issues_controller.rb

@@ -46,6 +46,8 @@ class Projects::IssuesController < Projects::ApplicationController
     push_frontend_feature_flag(:vue_issuable_sidebar, project.group)
   end
 
+  around_action :allow_gitaly_ref_name_caching, only: [:discussions]
+
   respond_to :html
 
   alias_method :designs, :show

changelogs/unreleased/cache-ref-names-in-discussion-endpoint.yml

+---
+title: Reduce Gitaly calls needed for issue discussions
+merge_request:
+author:
+type: performance