Rely on #overflow? for determining oversized MR

ee/lib/gitlab/code_owners.rb

@@ -41,7 +41,7 @@ module Gitlab
       #   the MUCH slower method of using Repository#diff_stats, which isn't
       #   subject to the same limit.
       #
-      if merge_request.diff_size == "1000+"
+      if oversized_merge_request?(merge_request, merge_request_diff)
         slow_path_lookup(merge_request, merge_request_diff)
       else
         fast_path_lookup(merge_request, merge_request_diff)
@@ -49,6 +49,12 @@ module Gitlab
     end
     private_class_method :paths_for_merge_request
 
+    def self.oversized_merge_request?(merge_request, merge_request_diff)
+      mrd_to_check_for_overflow = merge_request_diff || merge_request.merge_request_diff
+
+      mrd_to_check_for_overflow.overflow?
+    end
+
     def self.slow_path_lookup(merge_request, merge_request_diff)
       merge_request_diff = merge_request_diff || merge_request.merge_request_diff
 

ee/spec/lib/gitlab/code_owners_spec.rb

@@ -64,7 +64,7 @@ describe Gitlab::CodeOwners do
     it "return equivalent results" do
       fast_results = described_class.entries_for_merge_request(merge_request).first
 
-      expect(merge_request).to receive(:diff_size).and_return("1000+")
+      expect(merge_request.merge_request_diff).to receive(:overflow?).and_return(true)
 
       slow_results = described_class.entries_for_merge_request(merge_request).first
 
@@ -113,7 +113,7 @@ describe Gitlab::CodeOwners do
 
       context 'when the merge request is large (>1_000 files)' do
         before do
-          expect(merge_request).to receive(:diff_size).and_return("1000+")
+          expect(merge_request.merge_request_diff).to receive(:overflow?).and_return(true)
         end
 
         it 'generates paths via .slow_path_lookup' do