Follow-up to r1514315: Avoid potential issue with '%' character in
SVN_VER_NUMBER or SVN_BUILD_TARGET.

Found by: danielsh

* subversion/libsvn_ra_serf/serf.c
  (get_user_agent_string): Do not use SVN_VER_NUMBER and SVN_BUILD_TARGET in 
   format string.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1514628 13f79535-47bb-0310-9956-ffa450edef68