Fix package indices migration for MySQL

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

ee/db/migrate/20180810155213_add_more_indices_to_packages.rb

@@ -7,11 +7,17 @@ class AddMoreIndicesToPackages < ActiveRecord::Migration
 
   def up
     add_concurrent_index :packages_package_files, [:package_id, :file_name]
-    add_concurrent_index :packages_maven_metadata, :path
+    add_concurrent_index :packages_maven_metadata, :path, length: text_index_length
   end
 
   def down
     remove_concurrent_index :packages_package_files, [:package_id, :file_name]
-    remove_concurrent_index :packages_maven_metadata, :path
+    remove_concurrent_index :packages_maven_metadata, :path, length: text_index_length
+  end
+
+  private
+
+  def text_index_length
+    Gitlab::Database.mysql? ? 20 : nil
   end
 end