Add fallback path lookup for large MRs

What does this MR do?

Since MergeRequest#modified_paths is capped at returning 1000 records, we will have an incomplete listing of the filepaths for MRs with >1000 files. This can lead to CODEOWNERS rules not being consistently applied. Here, we get around this by adding a fallback to the much slower Repository#diff_stats approach to generate this data. This will, of course, result in slightly slower CODEOWNERS processing for these large MRs, but is the only way to ensure that we're fulfilling the service promise of the feature.

Closes #34118 (closed)