Followup to r1578853, optimize svnserve to only get properties from
the FS layer when needed.

* subversion/svnserve/serve.c
  (get_file, get_dir): Only build property hashes when the properties
   are going to be sent.

* subversion/libsvn_ra_svn/protocol: Add get-dir and get-file NOTE.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1578875 13f79535-47bb-0310-9956-ffa450edef68