dir.c: relax base option
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59476 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
dir.c: relax base option
or an empty string as :base option. [Feature #13056]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59476 b2dd03c8-39d4-4d8f-98ff-823fe69b080e