Partly redo r1072299 as it minimizes the APR overhead for small buffers on Win32 and document the rationales. Also, handle the case that APR might fail *after* writing some data.
Partly redo r1072299 as it minimizes the APR overhead for small buffers
on Win32 and document the rationales. Also, handle the case that APR might
fail *after* writing some data.

* subversion/libsvn_subr/io.c
  (svn_io_file_write_full): optimize & document

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion