Fix potential FSFS transaction corruption issue. The issue could happen 
only if processes are writing to the same transaction and using extremely 
large path.

* subversion/libsvn_fs_fs/low_level.c
  (write_change_entry): Collect all change info in stringbuf and write in one
   atomic write call.
* subversion/libsvn_fs_fs/transaction.c
  (svn_fs_fs__add_change): Remove APR_BUFFERED flag when opening changes 
   file for appending.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1519624 13f79535-47bb-0310-9956-ffa450edef68