Factor out a generic pointer array deep copy function from the 
svn_rangelist_dup code.

* subversion/libsvn_subr/mergeinfo.c
  (ptr_array_dup): New function allowing for array elements of
                   arbitrary but uniform size.  Code mainly 
                   taken from ... 
  (svn_rangelist_dup): ... here.  Call the new function.

Patch by: julianfoad

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1570904 13f79535-47bb-0310-9956-ffa450edef68