Fixed source branch name not being in new merge request dropdown toggle

Closes #29660

app/views/projects/merge_requests/_new_compare.html.haml

@@ -21,7 +21,7 @@
                   selected: f.object.source_project_id
           .merge-request-select.dropdown
             = f.hidden_field :source_branch
-            = dropdown_toggle local_assigns.fetch(f.object.source_branch, "Select source branch"), { toggle: "dropdown", field_name: "#{f.object_name}[source_branch]" }, { toggle_class: "js-compare-dropdown js-source-branch" }
+            = dropdown_toggle f.object.source_branch || "Select source branch", { toggle: "dropdown", field_name: "#{f.object_name}[source_branch]" }, { toggle_class: "js-compare-dropdown js-source-branch" }
             .dropdown-menu.dropdown-menu-selectable.dropdown-source-branch
               = dropdown_title("Select source branch")
               = dropdown_filter("Search branches")

spec/features/merge_requests/create_new_mr_spec.rb

@@ -46,6 +46,12 @@ feature 'Create New Merge Request', feature: true, js: true do
     end
   end
 
+  it 'populates source branch button' do
+    visit new_namespace_project_merge_request_path(project.namespace, project, change_branches: true, merge_request: { target_branch: 'master', source_branch: 'fix' })
+
+    expect(find('.js-source-branch')).to have_content('fix')
+  end
+
   it 'allows to change the diff view' do
     visit new_namespace_project_merge_request_path(project.namespace, project, merge_request: { target_branch: 'master', source_branch: 'fix' })