Allow creating merge request even if target branch is not specified in query params

Closes #28890

app/services/merge_requests/build_service.rb

@@ -6,7 +6,7 @@ module MergeRequests
       merge_request.source_project  = find_source_project
       merge_request.target_project  = find_target_project
       merge_request.target_branch   = find_target_branch
-      merge_request.can_be_created  = branches_valid? && source_branch_specified? && target_branch_specified?
+      merge_request.can_be_created  = branches_valid?
       compare_branches if branches_present?
       assign_title_and_description if merge_request.can_be_created

changelogs/unreleased/28890-allow-creating-mr-without-target-branch-in-url.yml

+---
+title: Allow creating merge request even if target branch is not specified in query
+  params
+merge_request: 9968
+author:

spec/features/merge_requests/user_uses_slash_commands_spec.rb

@@ -133,7 +133,6 @@ feature 'Merge Requests > User uses slash commands', feature: true, js: true do
       it 'changes target_branch in new merge_request' do
         visit new_namespace_project_merge_request_path(another_project.namespace, another_project, new_url_opts)
-        click_button "Compare branches and continue"
         fill_in "merge_request_title", with: 'My brand new feature'
         fill_in "merge_request_description", with: "le feature \n/target_branch fix\nFeature description:"

spec/services/merge_requests/build_service_spec.rb

@@ -49,10 +49,13 @@ describe MergeRequests::BuildService, services: true do
       let(:commits) { Commit.decorate([commit_1], project) }
       it 'creates compare object with target branch as default branch' do
-        expect(merge_request.can_be_created).to eq(false)
         expect(merge_request.compare).to be_present
         expect(merge_request.target_branch).to eq(project.default_branch)
       end
+
+      it 'allows the merge request to be created' do
+        expect(merge_request.can_be_created).to eq(true)
+      end
     end
     context 'same source and target branch' do