Collect #file_path info from #diff_files

What does this MR do?

@oswaldo spotted this one earlier today; none of our previous testing were exercising the #file_paths endpoint or otherwise covering up this bug. Only diff_files have #file_path -- diffs have only #new_path and #old_path, neither of which is what we're looking for. I believe this is an issue caused by a change to the code late in the development process, after we'd solidified our understanding of the data and had discarded any testing struts that would've caught this. Great example of where the "don't test private methods" rule and our assumptions about code behavior when setting up tests bit us.

Related to #39104 (closed)