Allow SpecRunner to not run in parallel

tasks/check_commit.rake

@@ -14,8 +14,12 @@ RuboCop::RakeTask.new(:check_commit, :commit) do |t, args|
   paths.reject { |p| p.start_with?(/docs|Gemfile|README|CHANGELOG/) }
   specs = paths.select { |p| p.start_with?('spec') }
 
-  puts "Checking: #{paths.join(' ')}"
-  RuboCop::SpecRunner.new(specs).run_specs
+  if specs.empty?
+    puts 'Caution: No spec was changed!'
+  else
+    puts "Checking: #{paths.join(' ')}"
+    RuboCop::SpecRunner.new(specs, parallel: false).run_specs
+  end
 
   t.patterns = paths
 end

tasks/spec_runner.rake

@@ -13,18 +13,20 @@ module RuboCop
   class SpecRunner
     attr_reader :rspec_args
 
-    def initialize(rspec_args = %w[spec], external_encoding: 'UTF-8', internal_encoding: nil)
+    def initialize(rspec_args = %w[spec], parallel: true,
+                   external_encoding: 'UTF-8', internal_encoding: nil)
       @rspec_args = rspec_args
       @previous_external_encoding = Encoding.default_external
       @previous_internal_encoding = Encoding.default_internal
 
       @temporary_external_encoding = external_encoding
       @temporary_internal_encoding = internal_encoding
+      @parallel = parallel
     end
 
     def run_specs
       n_failures = with_encoding do
-        if Process.respond_to?(:fork)
+        if @parallel && Process.respond_to?(:fork)
           parallel_runner_klass.new(rspec_args).execute
         else
           ::RSpec::Core::Runner.run(rspec_args)