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, 10 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/migration.rb b/activerecord/lib/active_record/migration.rb
index 63ec8f6745..1e870d4c44 100644
--- a/activerecord/lib/active_record/migration.rb
+++ b/activerecord/lib/active_record/migration.rb
@@ -477,6 +477,7 @@ module ActiveRecord
   class Migration
     autoload :CommandRecorder, 'active_record/migration/command_recorder'
 
+    MigrationFilenameRegexp = /\A([0-9]+)_([_a-z0-9]*)\.?([_a-z0-9]*)?\.rb\z/ # :nodoc:
 
     # This class is used to verify that all migrations have been run before
     # loading a web page if config.active_record.migration_error is set to :page_load
@@ -995,14 +996,21 @@ module ActiveRecord
         Array(@migrations_paths)
       end
 
+      def match_to_migration_filename?(filename) # :nodoc:
+        File.basename(filename) =~ Migration::MigrationFilenameRegexp
+      end
+
+      def parse_migration_filename(filename) # :nodoc:
+        File.basename(filename).scan(Migration::MigrationFilenameRegexp).first
+      end
+
       def migrations(paths)
         paths = Array(paths)
 
         files = Dir[*paths.map { |p| "#{p}/**/[0-9]*_*.rb" }]
 
         migrations = files.map do |file|
-          version, name, scope = file.scan(/([0-9]+)_([_a-z0-9]*)\.?([_a-z0-9]*)?\.rb\z/).first
-
+          version, name, scope = parse_migration_filename(file)
           raise IllegalMigrationNameError.new(file) unless version
           version = version.to_i
           name = name.camelize