dir.c: shortcut for case-insensitive name

* dir.c (glob_helper): shortcut for case-insensitive name by
  stopping reading after a matching name found.
  [ruby-core:63591] [Bug #10015]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48975 b2dd03c8-39d4-4d8f-98ff-823fe69b080e