Replace references to MergeRequestDiff#commits with st_commits when we care only about the number of commits

What does this MR do?

It optimizes the performance by replacing calls to MergeRequestDiff#commits with MergeRequestDiff#st_commits when it's possible. We don't have to instantiate all objects to check the size of the array

Does this MR meet the acceptance criteria?

• Changelog entry added
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together