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.