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, 2 insertions, 2 deletions

diff --git a/railties/test/isolation/abstract_unit.rb b/railties/test/isolation/abstract_unit.rb
index 6077ba3ee7..0fe62df8ba 100644
--- a/railties/test/isolation/abstract_unit.rb
+++ b/railties/test/isolation/abstract_unit.rb
@@ -301,7 +301,7 @@ module TestHelpers
     # stderr:: true to pass STDERR output straight to the "real" STDERR.
     #   By default, the STDERR and STDOUT of the process will be
     #   combined in the returned string.
-    def rails(*args, allow_failure: false, stderr: false, stdin: File::NULL)
+    def rails(*args, allow_failure: false, stderr: false)
       args = args.flatten
       fork = true
 
@@ -328,7 +328,7 @@ module TestHelpers
           out_read.close
           err_read.close if err_read
 
-          $stdin.reopen(stdin, "r")
+          $stdin.reopen(File::NULL, "r")
           $stdout.reopen(out_write)
           $stderr.reopen(err_write)