Add helper to support other libraries expecting diffs

lib/rspec/support/spec.rb

@@ -2,6 +2,7 @@
 require 'rspec/support/spec/in_sub_process'
 
 RSpec::Support.require_rspec_support "spec/deprecation_helpers"
+RSpec::Support.require_rspec_support "spec/diff_helpers"
 RSpec::Support.require_rspec_support "spec/with_isolated_stderr"
 RSpec::Support.require_rspec_support "spec/stderr_splitter"
 RSpec::Support.require_rspec_support "spec/formatting_support"

lib/rspec/support/spec/diff_helpers.rb

@@ -0,0 +1,29 @@
+require 'diff/lcs'
+
+module RSpec
+  module Support
+    module Spec
+      module DiffHelpers
+        # In the updated version of diff-lcs several diff headers change format slightly
+        # compensate for this and change minimum version in RSpec 4
+        if ::Diff::LCS::VERSION.to_f < 1.4
+          def one_line_header(line_number=2)
+            "-1,#{line_number} +1,#{line_number}"
+          end
+
+          def removing_two_line_header
+            "-1,3 +1"
+          end
+        else
+          def one_line_header(_=2)
+            "-1 +1"
+          end
+
+          def removing_two_line_header
+            "-1,3 +1,5"
+          end
+        end
+      end
+    end
+  end
+end

spec/rspec/support/differ_spec.rb

@@ -7,16 +7,7 @@
 module RSpec
   module Support
     RSpec.describe Differ do
-
-      # In the updated version of diff-lcs several diff headers change format slightly
-      # compensate for this and change minimum version in RSpec 4
-      if Diff::LCS::VERSION.to_f < 1.4
-        one_line_header = "-1,2 +1,2"
-        removing_two_line_header = "-1,3 +1"
-      else
-        one_line_header = "-1 +1"
-        removing_two_line_header = "-1,3 +1,5"
-      end
+      include Spec::DiffHelpers
 
       describe '#diff' do
         let(:differ) { RSpec::Support::Differ.new }