* file.c (rb_file_size, rb_file_flock): improve parformance of Winodws.

* file.c (rb_file_truncate): removed unnecessary #ifdef.

* test/test_file.rb (TestFile#test_truncate_size): added an assertion
  for File#size.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42577 b2dd03c8-39d4-4d8f-98ff-823fe69b080e