gc.c: build complete object graph for RGENGC_CHECK_MODE

	* gc.c (reflist_add): return 0 if reference already exists
	* gc.c (allrefs_add): return 1 on newly added references
	* gc.c (allrefs_i): follow references to construct complete object
	  graph. before this patch, RGENGC_CHECK could fail to verify some WB
	  miss issues. [Bug #9226] [ruby-core:58959]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44109 b2dd03c8-39d4-4d8f-98ff-823fe69b080e