Fix FSFS repository corruption on power or network disk failure on Windows:
http://svn.haxx.se/dev/archive-2013-05/0245.shtml

This is revert of r1082451.

* subversion/libsvn_subr/io.c
  (svn_io_write_unique): Perform svn_io_file_flush_to_disk() on all 
   platforms.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1483781 13f79535-47bb-0310-9956-ffa450edef68