Resolve "MR diffs file count increments while batch loading"

What does this MR do?

Updates the diffs app to use the file count returned from the API instead of calculating via "diff_files.length"

Why though?

The problem is that when we switch over to batch diff loading the array starts out with a length of 0 then increments with each subsequent batch load. That is not ideal.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

This could impact anything that was depending on the diffs getter diffFilesLength which should have only been in the diffs Vue app. I have searched far and wide and updated the instances I found.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Closes #55347 (closed)