Adjust order of verifications for CommitsController
What does this MR do and why?
Contributes to https://gitlab.com/gitlab-com/gl-infra/production-engineering/-/issues/27625
Problem
The previous merge request !206365 (merged) introduced graceful degradation for unauthenticated routes. But it had a problem with the execution order commits being loaded before auth. This is suboptimal.
Solution
Ensure that authentication is triggered before commits.
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