dir.c: glob performance

* dir.c (glob_make_pattern): restrict searching case-insensitive
  name from the filesystem to only last part, for the performance.
  [ruby-core:63591] [ruby-core:63591]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48972 b2dd03c8-39d4-4d8f-98ff-823fe69b080e