Rename to Diffing and move module into credentials command

Helpers is more for sharing between commands. Since `Diffing` is only
for credentials we should just keep it only for credentials.

Replaces "pretty" with diffing since the former is ambiguous, while
diffing captures what it does. `opt_in` seemed clunky so it's swapped
for the one-word enable.

railties/lib/rails/command/helpers/pretty_credentials.rb

@@ -1,55 +0,0 @@
-# frozen_string_literal: true
-
-require "fileutils"
-
-module Rails
-  module Command
-    module Helpers
-      module PrettyCredentials
-        Error = Class.new(StandardError)
-
-        def opt_in_pretty_credentials
-          unless already_answered? || already_opted_in?
-            answer = yes?("Would you like to make the credentials diff from git more readable in the future? [Y/n]")
-          end
-
-          opt_in! if answer
-          FileUtils.touch(tracker) unless answer.nil?
-        rescue Error
-          say("Couldn't setup git to prettify the credentials diff")
-        end
-
-        private
-          def already_answered?
-            tracker.exist?
-          end
-
-          def already_opted_in?
-            system_call("git config --get 'diff.rails_credentials.textconv'", accepted_codes: [0, 1])
-          end
-
-          def opt_in!
-            system_call("git config diff.rails_credentials.textconv 'bin/rails credentials:show'", accepted_codes: [0])
-
-            git_attributes = Rails.root.join(".gitattributes")
-            File.open(git_attributes, "a+") do |file|
-              file.write(<<~EOM)
-                config/credentials/*.yml.enc diff=rails_credentials
-                config/credentials.yml.enc diff=rails_credentials
-              EOM
-            end
-          end
-
-          def tracker
-            Rails.root.join("tmp", "rails_pretty_credentials")
-          end
-
-          def system_call(command_line, accepted_codes:)
-            result = system(command_line)
-            raise(Error) if accepted_codes.exclude?($?.exitstatus)
-            result
-          end
-      end
-    end
-  end
-end

railties/lib/rails/commands/credentials/credentials_command.rb

@@ -1,18 +1,19 @@
 # frozen_string_literal: true
 
+require "pathname"
 require "active_support"
 require "rails/command/helpers/editor"
-require "rails/command/helpers/pretty_credentials"
 require "rails/command/environment_argument"
-require "pathname"
 
 module Rails
   module Command
     class CredentialsCommand < Rails::Command::Base # :nodoc:
       include Helpers::Editor
-      include Helpers::PrettyCredentials
       include EnvironmentArgument
 
+      require_relative "credentials_command/diffing"
+      include Diffing
+
       self.environment_desc = "Uses credentials from config/credentials/:environment.yml.enc encrypted by config/credentials/:environment.key key"
 
       no_commands do
@@ -37,7 +38,7 @@ def edit
         end
 
         say "File encrypted and saved."
-        opt_in_pretty_credentials
+        enable_credentials_diffing
       rescue ActiveSupport::MessageEncryptor::InvalidMessage
         say "Couldn't decrypt #{content_path}. Perhaps you passed the wrong key?"
       end

railties/lib/rails/commands/credentials/credentials_command/diffing.rb

@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+require "fileutils"
+
+module Rails::Command::CredentialsCommand::Diffing # :nodoc:
+  class Error < StandardError; end
+
+  def enable_credentials_diffing
+    unless already_answered? || enabled?
+      answer = yes?("Would you like to make the credentials diff from git more readable in the future? [Y/n]")
+    end
+
+    enable if answer
+    FileUtils.touch(tracker) unless answer.nil?
+  rescue Error
+    say "Couldn't setup git to enable credentials diffing"
+  end
+
+  private
+    def already_answered?
+      tracker.exist?
+    end
+
+    def enabled?
+      system_call("git config --get 'diff.rails_credentials.textconv'", accepted_codes: [0, 1])
+    end
+
+    def enable
+      system_call("git config diff.rails_credentials.textconv 'bin/rails credentials:show'", accepted_codes: [0])
+
+      git_attributes = Rails.root.join(".gitattributes")
+      File.open(git_attributes, "a+") do |file|
+        file.write(<<~EOM)
+          config/credentials/*.yml.enc diff=rails_credentials
+          config/credentials.yml.enc diff=rails_credentials
+        EOM
+      end
+    end
+
+    def tracker
+      Rails.root.join("tmp", "rails_pretty_credentials")
+    end
+
+    def system_call(command_line, accepted_codes:)
+      result = system(command_line)
+      raise(Error) if accepted_codes.exclude?($?.exitstatus)
+      result
+    end
+end