Resolve "Gitlab merge request view crash when commiting a js sourcemap file"

What does this MR do?

1. Stop loading an intermediate collection object, which may perform line highlighting and caching, just to count the diff files in an MR.
2. When we have to load that intermediate collection object, don't try to highlight and cache files that aren't displayed due to entries in .gitattributes.

Does this MR meet the acceptance criteria?

• Changelog entry 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

What are the relevant issue numbers?

Closes #25931 (closed).