Make `db:migrate:status` to render `1_some.rb` format migrate files.

`1_valid_people_have_last_names.rb` and
`20150823202140_create_users.rb` are valid migration file name.
But `1_valid_people_have_last_names.rb` is rendered as
`********** NO FILE **********` when `rake db:migrate:status`.

Fix to this bug, this commit includes

* define some API private methdos and a Constant
  `match_to_migration_filename?`, `parse_migration_filename`, and
  `MigrationFilenameRegexp`
* use these methods in `db:migrate:status` task

Example:

These files are in `db/migrate`

* 1_valid_people_have_last_names.rb
* 20150819202140_irreversible_migration.rb
* 20150823202140_add_admin_flag_to_users.rb
* 20150823202141_migration_tests.rb
* 2_we_need_reminders.rb
* 3_innocent_jointable.rb

we can migrate all of them.

Before

```shell
$ bundle exec rake db:migrate:status

...

 Status   Migration ID    Migration Name
--------------------------------------------------
   up     001             ********** NO FILE **********
   up     002             ********** NO FILE **********
   up     003             ********** NO FILE **********
   up     20150819202140  Irreversible migration
   up     20150823202140  Add admin flag to users
   up     20150823202141  Migration tests
```

After

```shell
$ bundle exec rake db:migrate:status

...

 Status   Migration ID    Migration Name
--------------------------------------------------
   up     001             Valid people have last names
   up     002             We need reminders
   up     003             Innocent jointable
   up     20150819202140  Irreversible migration
   up     20150823202140  Add admin flag to users
   up     20150823202141  Migration tests
```

1 files changed, 7 insertions, 5 deletions

diff --git a/activerecord/lib/active_record/railties/databases.rake b/activerecord/lib/active_record/railties/databases.rake
index b6f3695856..5b1ea16f0b 100644
--- a/activerecord/lib/active_record/railties/databases.rake
+++ b/activerecord/lib/active_record/railties/databases.rake
@@ -100,12 +100,14 @@ db_namespace = namespace :db do
 
       file_list =
           ActiveRecord::Tasks::DatabaseTasks.migrations_paths.flat_map do |path|
-            # match "20091231235959_some_name.rb" and "001_some_name.rb" pattern
-            Dir.foreach(path).grep(/^(\d{3,})_(.+)\.rb$/) do
-              version = ActiveRecord::SchemaMigration.normalize_migration_number($1)
+            Dir.foreach(path).map do |file|
+              next unless ActiveRecord::Migrator.match_to_migration_filename?(file)
+
+              version, name, scope = ActiveRecord::Migrator.parse_migration_filename(file)
+              version = ActiveRecord::SchemaMigration.normalize_migration_number(version)
               status = db_list.delete(version) ? 'up' : 'down'
-              [status, version, $2.humanize]
-            end
+              [status, version, (name + scope).humanize]
+            end.compact
           end
 
       db_list.map! do |version|