Ignore configuration files outside of the project

If by any chance, one happens to leave a `.rubocop.yml` file outside of
my project, ignore it.

lib/rubocop/config_loader.rb

@@ -24,7 +24,7 @@ class << self
 
       attr_accessor :debug, :ignore_parent_exclusion,
                     :disable_pending_cops, :enable_pending_cops
-      attr_writer :default_configuration
+      attr_writer :default_configuration, :project_root
 
       alias debug? debug
       alias ignore_parent_exclusion? ignore_parent_exclusion
@@ -164,7 +164,7 @@ def merge_with_default(config, config_file, unset_nil: true)
       private
 
       def find_project_dotfile(target_dir)
-        find_file_upwards(DOTFILE, target_dir)
+        find_file_upwards(DOTFILE, target_dir, project_root)
       end
 
       def find_project_root

lib/rubocop/rspec/shared_contexts.rb

@@ -18,13 +18,14 @@
       ENV.delete('XDG_CONFIG_HOME')
 
       base_dir = example.metadata[:project_inside_home] ? virtual_home : tmpdir
-      working_dir = File.join(base_dir, 'work')
+      root = example.metadata[:root]
+      working_dir = root ? File.join(base_dir, 'work', root) : File.join(base_dir, 'work')
 
       # Make upwards search for .rubocop.yml files stop at this directory.
       RuboCop::FileFinder.root_level = working_dir
 
       begin
-        Dir.mkdir(working_dir)
+        FileUtils.mkdir_p(working_dir)
 
         Dir.chdir(working_dir) do
           example.run

spec/rubocop/config_loader_spec.rb

@@ -82,6 +82,26 @@
       end
     end
 
+    context 'when there is a spurious rubocop config outside of the project', root: 'dir' do
+      let(:dir_path) { 'dir' }
+
+      before do
+        # Force reload of project root
+        described_class.project_root = nil
+        create_empty_file('Gemfile')
+        create_empty_file('../.rubocop.yml')
+      end
+
+      after do
+        # Don't leak project root change
+        described_class.project_root = nil
+      end
+
+      it 'ignores the spurious config and falls back to the provided default file if run from the project' do
+        expect(configuration_file_for).to end_with('config/default.yml')
+      end
+    end
+
     context 'when a config file exists in the parent directory' do
       let(:dir_path) { 'dir' }