merge revision(s) 39811: [Backport #8146]

	* gc.c: Avoid unnecessary heap growth. patched by tmm1(Aman Gupta).
	  [Bug #8093] [ruby-core:53393]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@40176 b2dd03c8-39d4-4d8f-98ff-823fe69b080e