[PATCH] Timestamped migrations should detect collisions

The timestamp in timestamped migrations does not account for other migrations sharing the same timestamp. The corner case was previously pathological.

One reasonable use case is @James Adam 's suggestion for engine migrations in #2058 Rake tasks for run engine migrations. Briefly, engine migrations are copied into the host app. They are re-indexed at the same time, preserving their relative order while letting the host application keep a single, linear schema history.

This copy & re-index can add many migrations to an application at once, which ActiveRecord::Generators::Base.next_migration_number will naively timestamp to the same second.

One solution is to have next_migration_number push the returned timestamp 1 second into the future until there's no collision. I'll see if I can brew up a patch.