When a MergeRequestDiff is "overflowed" (contains more than a certain
number of files, 1,000 as of 12.10) #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
Gitlab::CodeOwners) This is less than ideal, as
Repository#diff_stats is slow.. like, super slow. Like, you shouldn't
use this unless you need to.

Since it is so slow, we really should have a single, unified
implementation of this fallback, and it should be here inside
MergeRequest#modifed_paths. The consumer shouldn't have to special
case their way around this limitation, and having it in 2 different
locations as of this comment, we really shouldn't have multiple places
calling their own implementations of code that is potentially very
slow. By adding this fallback option, flagged to NOT run, we can then
follow-on and remove our disparate and potentially divergent
implementations, and provide a better option for future use.