The purpose of this change is to allow copying fail on the same names.
Migrations change database and they should be treated with caution,
if 2 migrations are named the same it's much better to skip migration
and allow user decide if it should be copied or not.
ActiveRecord::Migration#copy allows to copy migrations from one place
to another, changing migrations versions and adding scope to filename.
For example:
ActiveRecord::Migration.copy("db/migrate",
:blog_engine => "vendor/gems/blog/db/migrate")
will copy all migrations from vendor/gems/blog/db/migrate to db/migrate
with such format:
Versions of copied migrations will be reversioned to be appended after
migrations that already exists in db/migrate
