* gc.c (heap_extend_pages): calculate next growing heap size.
* gc.c (heap_set_increment): accept addition pages instead of
  minimum pages.
* gc.c (gc_after_sweep): use heap_etend_pages().
* gc.c (gc_heap_prepare_minimum_pages): add only 1 page.
* gc.c (heap_ready_to_gc): add only 1 page.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45949 b2dd03c8-39d4-4d8f-98ff-823fe69b080e