Enable ref name caching for merge request diffs

In one example, we found
`Projects::MergeRequests::DiffsController#diffs_metadata` calling Gitaly
over 3000 times with the same ref name for FindCommit. Since all actions
in this controller are GET requests, it should be safe to enable this
for all actions inside `DiffsController`, just as we do with
`CommitsController`.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/217192

app/controllers/projects/merge_requests/diffs_controller.rb

@@ -9,6 +9,8 @@ class Projects::MergeRequests::DiffsController < Projects::MergeRequests::Applic
   before_action :define_diff_vars
   before_action :define_diff_comment_vars, except: [:diffs_batch, :diffs_metadata]
 
+  around_action :allow_gitaly_ref_name_caching
+
   def show
     render_diffs
   end

changelogs/unreleased/sh-enable-ref-caching-diffs-controller.yml

+---
+title: Enable ref name caching for merge request diffs
+merge_request: 31530
+author:
+type: performance