Optimize 'svn mergeinfo --showrevs' command a bit.

* subversion/svn/mergeinfo-cmd.c
  (svn_cl__mergeinfo): Do not request revision properties since we print 
   revision numbers only.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1492005 13f79535-47bb-0310-9956-ffa450edef68