Avoid repeating a string literal.

* subversion/mod_dav_svn/mod_dav_svn.c
  (NO_MAP_TO_STORAGE_NOTE): New macro.
  (dav_svn__translate_name, dav_svn__map_to_storage): Use macro.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1513472 13f79535-47bb-0310-9956-ffa450edef68