Called during mergeinfo parsing and others,  duplicating rangelists
can use some more tuning.

* subversion/libsvn_subr/mergeinfo.c
  (svn_rangelist_dup): minimize the loop body


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1483310 13f79535-47bb-0310-9956-ffa450edef68