diff --git a/lib/gitlab/git/repository.rb b/lib/gitlab/git/repository.rb
index 6cae0fc1780548dab7d0c84e0c6b788a239c72f2..ed45d3eb0306d894330d1610ee8f7cce56ef645a 100644
--- a/lib/gitlab/git/repository.rb
+++ b/lib/gitlab/git/repository.rb
@@ -518,13 +518,15 @@ def diff_stats(left_id, right_id)
         empty_diff_stats
       end
 
-      def find_changed_paths(commits)
+      def find_changed_paths(commits, merge_commit_diff_mode: nil)
         processed_commits = commits.reject { |ref| ref.blank? || Gitlab::Git.blank_ref?(ref) }
 
         return [] if processed_commits.empty?
 
         wrapped_gitaly_errors do
-          gitaly_commit_client.find_changed_paths(processed_commits)
+          gitaly_commit_client.find_changed_paths(
+            processed_commits, merge_commit_diff_mode: merge_commit_diff_mode
+          )
         end
       rescue CommandError, TypeError, NoRepository
         []
diff --git a/spec/lib/gitlab/git/repository_spec.rb b/spec/lib/gitlab/git/repository_spec.rb
index 5e3b136cf2e53402539c68019d4841c18642ac58..b137157f2d5d6d93f96d5c44e6f7dda2a69b0da3 100644
--- a/spec/lib/gitlab/git/repository_spec.rb
+++ b/spec/lib/gitlab/git/repository_spec.rb
@@ -1675,6 +1675,41 @@ def create_commit(blobs)
       expect(collection).to be_a(Enumerable)
       expect(collection.to_a).to be_empty
     end
+
+    describe 'merge_commit_diff_mode argument' do
+      let(:gitaly_commit_client) { double('Gitlab::GitalyClient::CommitService') }
+
+      before do
+        allow(repository).to receive(:gitaly_commit_client).and_return(gitaly_commit_client)
+        allow(gitaly_commit_client).to receive(:find_changed_paths)
+      end
+
+      context 'when omitted' do
+        before do
+          repository.find_changed_paths(['sha'])
+        end
+
+        it 'defaults to nil' do
+          expect(gitaly_commit_client)
+            .to have_received(:find_changed_paths)
+            .with(['sha'], merge_commit_diff_mode: nil)
+        end
+      end
+
+      context 'when included' do
+        let(:passed_value) { 'foobar' }
+
+        before do
+          repository.find_changed_paths(['sha'], merge_commit_diff_mode: passed_value)
+        end
+
+        it 'passes the value on to the commit client' do
+          expect(gitaly_commit_client)
+            .to have_received(:find_changed_paths)
+            .with(['sha'], merge_commit_diff_mode: passed_value)
+        end
+      end
+    end
   end
 
   describe "#ls_files" do