Example:
  ActiveRecord::Migrator.migrate(path) do |migration|
    migration.name =~ /User/
  end

The above example will migrate only migrations with User in
the name

With this commit, ActiveRecord will also look for migrations
in db/migrate subdirectories.

49ebe51 fixed copying migrations, but existing migrations would still
trigger warnings. The proper way to compare migrations is to ignore
origin lines - if migration is identical it means that we can
silently skip it, regardless where it comes from.

There was a bug in ActiveRecord::Migration.copy method, which
prevented adding special comment about the origin of migration.

Because of that, the check if migration is identical or if it's
not and should be skipped was always saying that migration is
skipped, which was causing additional useless warnings about
skipped migrations.

This reverts commit 0e407a90, reversing
changes made to 533a9f84.

Conflicts:

	activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
	activerecord/test/cases/migration_test.rb

This reverts commit 81fad6a2, reversing
changes made to 23101de2.

Conflicts:

	activerecord/test/cases/migration_test.rb