dir.c: fix Dir.glob starts with brace

	* dir.c (ruby_glob0): expand braces if a glob pattern starts with
	  brace.
	  [ruby-core:91728] [Bug #15649]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67353 b2dd03c8-39d4-4d8f-98ff-823fe69b080e