Another cheap mergeinfo retrieval optimization.

* subversion/libsvn_client/merge.c
  (short_circuit_mergeinfo_log): Do not request revision properties since we
   don't need them.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1492020 13f79535-47bb-0310-9956-ffa450edef68