Add specs for exporting merge requests as diff or patch

spec/factories.rb

@@ -63,7 +63,22 @@ FactoryGirl.define do
       closed true
     end
 
+    # pick 3 commits "at random" (from bcf03b5d~3 to bcf03b5d)
+    trait :with_diffs do
+      target_branch "bcf03b5d~3"
+      source_branch "bcf03b5d"
+      st_commits do
+        [Commit.new(project.repo.commit('bcf03b5d')),
+         Commit.new(project.repo.commit('bcf03b5d~1')),
+         Commit.new(project.repo.commit('bcf03b5d~2'))]
+      end
+      st_diffs do
+        project.repo.diff("bcf03b5d~3", "bcf03b5d")
+      end
+    end
+
     factory :closed_merge_request, traits: [:closed]
+    factory :merge_request_with_diffs, traits: [:with_diffs]
   end
 
   factory :note do