Realign retab.rb
Make retab.rb smarter about ruby file types

Instead of just relying on a filename of *.rb, use the file utility to
determine file type.

For systems that lack lack 'which' and 'file', fall back to filename
matching.

This is useful for retabbing things like 'msfconsole' that don't have a
.rb extension.
Short circut file checking if it's .rb

Makes things a little faster.
Land #2364 - Update retab util