Never set special MR titles for external issues

app/services/merge_requests/build_service.rb

@@ -154,13 +154,9 @@ module MergeRequests
     end
 
     def assign_title_from_issue
-      return unless issue
+      return unless issue && issue.is_a?(Issue)
 
-      merge_request.title =
-        case issue
-        when Issue         then "Resolve \"#{issue.title}\""
-        when ExternalIssue then merge_request.source_branch.titleize.humanize
-        end
+      merge_request.title = "Resolve \"#{issue.title}\""
     end
 
     def issue_iid

spec/services/merge_requests/build_service_spec.rb

@@ -171,6 +171,24 @@ describe MergeRequests::BuildService do
           end
         end
       end
+
+      context 'branch starts with external issue IID followed by a hyphen' do
+        let(:source_branch) { '12345-fix-issue' }
+
+        before do
+          allow(project).to receive(:external_issue_tracker).and_return(true)
+        end
+
+        it 'uses the title of the commit as the title of the merge request' do
+          expect(merge_request.title).to eq(commit_1.safe_message.split("\n").first)
+        end
+
+        it 'uses the description of the commit as the description of the merge request and appends the closes text' do
+          commit_description = commit_1.safe_message.split(/\n+/, 2).last
+
+          expect(merge_request.description).to eq("#{commit_description}\n\nCloses #12345")
+        end
+      end
     end
 
     context 'more than one commit in the diff' do
@@ -244,6 +262,10 @@ describe MergeRequests::BuildService do
         it 'sets the title to the humanized branch title' do
           expect(merge_request.title).to eq('12345 fix issue')
         end
+
+        it 'appends the closes text' do
+          expect(merge_request.description).to eq('Closes #12345')
+        end
       end
     end