Improve svn patch performance significantly.

* subversion/libsvn_diff/parse-diff.c
  (svn_diff_open_patch_file): Open patch file with APR_BUFFERED flag.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1486915 13f79535-47bb-0310-9956-ffa450edef68