Preload commits markdown cache

What does this MR do?

This is for improving the performance of Projects::MergeRequestsController#commits action.

Prevents n+1 requests to Redis when getting the value from cache on render if the cache is warm.

Previously, it'll make an HMGET request to Redis per commit which can result to 100 requests when viewing the commits in a MR. Now, a single Redis pipeline is used via with_markdown_cache.

Utilizes the #preload_markdown_cache! introduced in !35312 (merged).

Improvements

Tested on local development environment with RAILS_PROFILE=true and cache is warm.

• Before (average of 5 requests): 1.472014
• After (average of 5 requests): 1.08282

That's around 26% improvement.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

#209912