Properly handle "" directory in svn_io_start_cmd3(). Many callers already
expect the broken current handling for these paths, but at least new callers
can just pass canonical paths now.

* subversion/libsvn_subr/io.c
  (svn_io_start_cmd3): Handle "" directory.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1604761 13f79535-47bb-0310-9956-ffa450edef68