Add fallback option for overflowed MRDs

When a MergeRequestDiff is "overflowed" (contains more than a certain number of files, 1,000 as of %12.10) MergeRequest#modified_paths will cut-off its results at the last file of the limit (IE the 1,000th file.) For the majority of MRs, this is fine, and we often have fallback code to use Repository#diff_stats to get at the expanded list (for example, in lib/gitlab/code_owners.rb) This is less than ideal, as Repository#diff_stats is slow.. like, super slow. Like, you shouldn't use this unless you need to.