Fix incorrect default merge request title when external issue tracker is activated

app/services/merge_requests/build_service.rb

@@ -159,7 +159,7 @@ module MergeRequests
       merge_request.title =
         case issue
         when Issue         then "Resolve \"#{issue.title}\""
-        when ExternalIssue then "Resolve #{issue.title}"
+        when ExternalIssue then merge_request.source_branch.titleize.humanize
         end
     end

changelogs/unreleased/36669-default-mr-title-with-external-issues.yml

+---
+title: Default merge request title is set correctly again when external issue tracker is activated
+merge_request: 
+author:
+type: fixed

spec/services/merge_requests/build_service_spec.rb

@@ -241,8 +241,8 @@ describe MergeRequests::BuildService do
           allow(project).to receive(:external_issue_tracker).and_return(true)
         end
-        it 'sets the title to: Resolves External Issue $issue-iid' do
-          expect(merge_request.title).to eq('Resolve External Issue 12345')
+        it 'sets the title to the humanized branch title' do
+          expect(merge_request.title).to eq('12345 fix issue')
         end
       end
     end