Fix a bug of before_sha being inproperly evaluated to `checkout_sha`

lib/gitlab/ci/pipeline/chain/command.rb

@@ -46,7 +46,7 @@ module Gitlab
           end
 
           def before_sha
-            checkout_sha || before_sha || Gitlab::Git::BLANK_SHA
+            before_sha || checkout_sha || Gitlab::Git::BLANK_SHA
           end
 
           def protected_ref?
@@ -54,10 +54,6 @@ module Gitlab
               project.protected_for?(ref)
             end
           end
-
-          def error(message)
-            pipeline.errors.add(:base, message)
-          end
         end
       end
     end