* gc.c: change full GC timing to keep lower memory usage.
  Extend heap only at
  (1) after major GC
  or
  (2) after several (two times, at current) minor GC
  Details in https://bugs.ruby-lang.org/issues/9607#note-9
  [Bug #9607]



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46387 b2dd03c8-39d4-4d8f-98ff-823fe69b080e