* 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

Conflicts:
	ChangeLog
* 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

Conflicts:
	ChangeLog
	gc.c
Merge pull request #19 from github/2.1-gc-heap-growth

Backport 2.1 gc heap growth patch