Optimize creation of commit API by using Repository#commit instead of Repository#commits

Repository#commits is expensive because it has to use Rugged to walk the Git
tree as opposed to doing a direct ref lookup.

Improves performance in #34533

changelogs/unreleased/sh-optimize-project-commit-api.yml

+---
+title: Optimize creation of commit API by using Repository#commit instead of Repository#commits
+merge_request:
+author:

lib/api/commits.rb

@@ -67,7 +67,7 @@ module API
         result = ::Files::MultiService.new(user_project, current_user, attrs).execute
         if result[:status] == :success
-          commit_detail = user_project.repository.commits(result[:result], limit: 1).first
+          commit_detail = user_project.repository.commit(result[:result])
           present commit_detail, with: Entities::RepoCommitDetail
         else
           render_api_error!(result[:message], 400)