Don't add project_mirror_data columns that have already been added

In !5586, some `add_column` statements were moved between migrations.
If the original migrations were already run on a user's instance,
the new migrations would try to add the columns again.

db/migrate/20180502122856_create_project_mirror_data.rb

@@ -5,9 +5,9 @@ class CreateProjectMirrorData < ActiveRecord::Migration
 
   def up
     if table_exists?(:project_mirror_data)
-      add_column :project_mirror_data, :status, :string
-      add_column :project_mirror_data, :jid, :string
-      add_column :project_mirror_data, :last_error, :text
+      add_column :project_mirror_data, :status, :string unless column_exists?(:project_mirror_data, :status)
+      add_column :project_mirror_data, :jid, :string unless column_exists?(:project_mirror_data, :jid)
+      add_column :project_mirror_data, :last_error, :text unless column_exists?(:project_mirror_data, :last_error)
     else
       create_table :project_mirror_data do |t|
         t.references :project, index: true, foreign_key: { on_delete: :cascade }

ee/db/migrate/20170509153720_create_project_mirror_data_ee.rb

@@ -9,10 +9,10 @@ class CreateProjectMirrorDataEE < ActiveRecord::Migration
     # When moving from CE to EE, project_mirror_data may already exist, but will
     # not have all the required columns.
     if table_exists?(:project_mirror_data)
-      add_column_with_default :project_mirror_data, :retry_count, :integer, default: 0, allow_null: false
-      add_column :project_mirror_data, :last_update_started_at, :datetime_with_timezone
-      add_column :project_mirror_data, :last_update_scheduled_at, :datetime_with_timezone
-      add_column :project_mirror_data, :next_execution_timestamp, :datetime_with_timezone
+      add_column_with_default :project_mirror_data, :retry_count, :integer, default: 0, allow_null: false unless column_exists?(:project_mirror_data, :retry_count)
+      add_column :project_mirror_data, :last_update_started_at, :datetime_with_timezone unless column_exists?(:project_mirror_data, :last_update_started_at)
+      add_column :project_mirror_data, :last_update_scheduled_at, :datetime_with_timezone unless column_exists?(:project_mirror_data, :last_update_scheduled_at)
+      add_column :project_mirror_data, :next_execution_timestamp, :datetime_with_timezone unless column_exists?(:project_mirror_data, :next_execution_timestamp)
     else
       execute <<-SQL
         CREATE TABLE project_mirror_data

ee/db/migrate/20180502124117_add_missing_columns_to_project_mirror_data.rb

@@ -10,8 +10,8 @@ class AddMissingColumnsToProjectMirrorData < ActiveRecord::Migration
   disable_ddl_transaction!
 
   def up
-    add_column :project_mirror_data, :last_update_at, :datetime_with_timezone
-    add_column :project_mirror_data, :last_successful_update_at, :datetime_with_timezone
+    add_column :project_mirror_data, :last_update_at, :datetime_with_timezone unless column_exists?(:project_mirror_data, :last_update_at)
+    add_column :project_mirror_data, :last_successful_update_at, :datetime_with_timezone unless column_exists?(:project_mirror_data, :last_successful_update_at)
   end
 
   def down