This method almotst same as File.dirname(__FILE__). One
different behavior is it returns nil when __FILE__ returns nil.
[Feature #3346]
* NEWS: ditto
* test/ruby/test_method.rb: related test.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37432 b2dd03c8-39d4-4d8f-98ff-823fe69b080e