string.c: shrink too big buffer

	* string.c (rb_str_resize): shrink the buffer even if new length
  is same but it is enough smaller than the capacity.
	* file.c (expand_path): shrink expanded path which no longer needs
	  rooms to append.  [ruby-core:63114] [Bug #9934]

	* string.c (rb_str_resize): should consider the capacity instead
	  of the old length, as pointed out by nagachika.

	* string.c (rb_str_resize): update capa only when buffer get
	  reallocated.
	  http://d.hatena.ne.jp/nagachika/20140613/ruby_trunk_changes_46413_46420#r46413


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@47399 b2dd03c8-39d4-4d8f-98ff-823fe69b080e