Enable Gitaly FindCommit caching for TreeController

Accessing /namespace/project/tree/master appears to query
FindCommit 5 times with identical parameters. This reduces
the number of FindCommit queries to 1.

app/controllers/projects/tree_controller.rb

@@ -6,6 +6,8 @@ class Projects::TreeController < Projects::ApplicationController
   include CreatesCommit
   include ActionView::Helpers::SanitizeHelper
+  around_action :allow_gitaly_ref_name_caching, only: [:show]
+
   before_action :require_non_empty_project, except: [:new, :create]
   before_action :assign_ref_vars
   before_action :assign_dir_vars, only: [:create_dir]

changelogs/unreleased/sh-add-gitaly-ref-name-caching-tree-controller.yml

+---
+title: Enable Gitaly FindCommit caching for TreeController
+merge_request: 27100
+author:
+type: performance

spec/controllers/projects/tree_controller_spec.rb

@@ -16,6 +16,8 @@ describe Projects::TreeController do
     render_views
     before do
+      expect(::Gitlab::GitalyClient).to receive(:allow_ref_name_caching).and_call_original
+
       get(:show,
           params: {
             namespace_id: project.namespace.to_param,