Fix generating ignore list when one exists already

Generating an ignore list is done by running Rubocop over all the files
in the project to detect which have errors, and then writing that list
of files to `.standard_todo.yml`.

When a file was already ignored in `.standard_todo.yml`, Rubocop would
be configured to not run cops against it. In that case, no errors were
reported, so the file would not be written to `.standard_todo.yml`.

This leads to a situation where if a file is ignored, running
`standardrb --generated-todo` would remove it from the todo list, and
add all other files with errors. Running the same command again would
then flip the file list.

We fix this by setting the list of ignored files to an empty array
before running Rubocop in the `genignore` runner.

lib/standard/runners/genignore.rb

@@ -13,6 +13,7 @@ def call(config)
           config.rubocop_options[:formatters] = [["files", temp_file.path]]
           config.rubocop_options[:format] = "files"
           config.rubocop_options[:out] = temp_file.path
+          remove_project_files_from_ignore_list(config)
           Runners::Rubocop.new.call(config)
 
           # Read in the files with errors.  It will have the absolute paths
@@ -43,6 +44,14 @@ def call(config)
           temp_file.unlink
         end
       end
+
+      # FIXME: This will also remove files which are in
+      # `Standard::CreatesConfigStore::ConfiguresIgnoredPaths::DEFAULT_IGNORES`.
+      def remove_project_files_from_ignore_list(config)
+        options_config = config.rubocop_config_store.instance_variable_get("@options_config")
+        options_config["AllCops"] ||= []
+        options_config["AllCops"]["Exclude"] = []
+      end
     end
   end
 end

test/standard/runners/genignore_test.rb

@@ -30,6 +30,17 @@ def test_todo_generated
   def create_config(config_path)
     store = RuboCop::ConfigStore.new.tap do |config_store|
       config_store.options_config = config_path
+
+      # Simulate a `.standard_todo.yml` that contains an ignore file:
+      #
+      #   ---
+      #   ignore:
+      #     - errors_one.rb
+      #
+      options_config = config_store.instance_variable_get("@options_config")
+      options_config["AllCops"] ||= []
+      options_config["AllCops"]["Exclude"] ||= []
+      options_config["AllCops"]["Exclude"] |= [path("errors_one.rb")]
     end
 
     Standard::Config.new(nil, ["."], {}, store)