Revise credentials diffing flow to use a separate diff command

Didn't like the complicated stuff that happened on credentials:edit. It
would append to .gitattributes multiple times. Though I see why it was
written that way.

I'm cutting off for now, but since this new flow would require each developer
to run --enable perhaps this should really be:

1. Developer enrolls Rails app by running `credentials:diff --enable`
2. credentials:edit checks .gitattributes for `diff=rails_credentials` and
   if the current file is covered by that.
3. If so, set up the "rails_credentials" driver automatically.

1 files changed, 13 insertions, 30 deletions

diff --git a/railties/lib/rails/commands/credentials/credentials_command/diffing.rb b/railties/lib/rails/commands/credentials/credentials_command/diffing.rb
index 1598ecaa8d..b7330178a3 100644
--- a/railties/lib/rails/commands/credentials/credentials_command/diffing.rb
+++ b/railties/lib/rails/commands/credentials/credentials_command/diffing.rb
@@ -3,45 +3,28 @@
 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]")
+  def enable_diffing
+    if enabled?
+      say "Already enabled!"
+    else
+      enable
+      say "Diffing enabled! Editing a credentials file will display a diff of what actually changed."
     end
-
-    enable if answer
-    FileUtils.touch(tracker) unless answer.nil?
   rescue Error
-    say "Couldn't setup git to enable credentials diffing"
+    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])
+      system "git config --get diff.rails_credentials.textconv", out: File::NULL
     end
 
     def enable
-      system_call("git config diff.rails_credentials.textconv 'bin/rails credentials:diff'", 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
+      raise Error unless system("git config diff.rails_credentials.textconv 'bin/rails credentials:diff'")
 
-    def system_call(command_line, accepted_codes:)
-      result = system(command_line)
-      raise(Error) if accepted_codes.exclude?($?.exitstatus)
-      result
+      Rails.root.join(".gitattributes").write(<<~end_of_template, mode: "a")
+        config/credentials/*.yml.enc diff=rails_credentials
+        config/credentials.yml.enc diff=rails_credentials
+      end_of_template
     end
 end