Avoid N+1 calls for image_path when rendering commits

When rendering `projects/commits/commit` partial, it calls some
helper methods like `sprite_icon` and `clipboard_button`.

These methods call `sprite_icon_path` internally which is not that
slow (averaging around 6ms per call on my local machine). But when
called multiple times (e.g. rendering 100 commits), it adds up.

Some benchmark numbers for `Projects::MergeRequestsController#commits`
(average of 5 requests, with `RAILS_PROFILE` env var set to `true`:

- BEFORE: 1.051528
- AFTER: 0.684848

This shows around 35% improvement.

app/helpers/icons_helper.rb

@@ -28,11 +28,13 @@ module IconsHelper
   end
 
   def sprite_icon_path
+    @sprite_icon_path ||= begin
       # SVG Sprites currently don't work across domains, so in the case of a CDN
       # we have to set the current path deliberately to prevent addition of asset_host
       sprite_base_url = Gitlab.config.gitlab.url if ActionController::Base.asset_host
       ActionController::Base.helpers.image_path('icons.svg', host: sprite_base_url)
     end
+  end
 
   def sprite_file_icons_path
     # SVG Sprites currently don't work across domains, so in the case of a CDN

changelogs/unreleased/209912-memoize-sprites-icon-path.yml

+---
+title: Avoid N+1 calls for image_path when rendering commits
+merge_request: 36724
+author:
+type: performance

spec/helpers/icons_helper_spec.rb

@@ -22,8 +22,13 @@ RSpec.describe IconsHelper do
 
   describe 'sprite_icon_path' do
     it 'returns relative path' do
-      expect(sprite_icon_path)
-        .to eq icons_path
+      expect(sprite_icon_path).to eq(icons_path)
+    end
+
+    it 'only calls image_path once when called multiple times' do
+      expect(ActionController::Base.helpers).to receive(:image_path).once.and_call_original
+
+      2.times { sprite_icon_path }
     end
 
     context 'when an asset_host is set in the config it will return an absolute local URL' do