add migrate command for migrating renamed

Library/Homebrew/cmd/migrate.rb

+require "migrator"
+require "formula_renames"
+
+module Homebrew
+  def migrate
+    raise FormulaUnspecifiedError if ARGV.named.empty?
+
+    ARGV.resolved_formulae.each do |f|
+      if f.oldname
+        unless (rack = HOMEBREW_CELLAR/f.oldname).exist? && !rack.subdirs.empty?
+          raise NoSuchKegError, f.oldname
+        end
+        raise "#{rack} is a symlink" if rack.symlink?
+      end
+
+      migrator = Migrator.new(f)
+      migrator.migrate
+    end
+  end
+end