Fix regression in Merge request form

Merge request !7163 caused an error message to appear on the Merge
Request form when it was first loaded by the user. This commit fixes
that regression bug.

app/services/merge_requests/build_service.rb

@@ -48,11 +48,11 @@ module MergeRequests
       end
 
       # See if source and target branches exist
-      unless merge_request.source_project.commit(merge_request.source_branch)
+      if merge_request.source_branch.present? && !merge_request.source_project.commit(merge_request.source_branch)
         messages << "Source branch \"#{merge_request.source_branch}\" does not exist"
       end
 
-      unless merge_request.target_project.commit(merge_request.target_branch)
+      if merge_request.target_branch.present? && !merge_request.target_project.commit(merge_request.target_branch)
         messages << "Target branch \"#{merge_request.target_branch}\" does not exist"
       end
 

changelogs/unreleased/24627-fix-bad-mr-error-message.yml

+---
+title: Fix regression causing bad error message to appear on Merge Request form
+merge_request: 7599
+author: Alex Sanford

spec/features/merge_requests/create_new_mr_spec.rb

@@ -77,4 +77,11 @@ feature 'Create New Merge Request', feature: true, js: true do
       expect(page).to have_content "6049019_460s.jpg"
     end
   end
+
+  # Isolates a regression (see #24627)
+  it 'does not show error messages on initial form' do
+    visit new_namespace_project_merge_request_path(project.namespace, project)
+    expect(page).not_to have_selector('#error_explanation')
+    expect(page).not_to have_content('The form contains the following error')
+  end
 end