Revision 1758269

Fix issue #4652 in a different way. r1758069 fixed it in libsvn_subr but as
Ivan Zhakov points out the root cause of the problem is that 'svn merge'
passes in invalid data to svn_client_merge5().

* subversion/svn/merge-cmd.c
  (run_merge): If the merge target path is a URL, return an error message.