Don't reject command-line arguments of the form ".@abc", where "abc" is a
peg specifier that may be empty.

Since r878062, this form has been rejected with the error

  svn: E125001: '@abc' is just a peg revision. Maybe try '@abc@' instead?

apparently as an unintended side effect of rejecting the potentially
ambiguous form "@abc". The fix is to move the check up to a higher level
function.

* subversion/include/svn_client.h,
  subversion/libsvn_client/cmdline.c
  (svn_client_args_to_target_array2): Reject an argument of the form "@abc".
    At this point the argument has not yet been converted to internal style.
    Document that an argument of the form "@abc" is an error.

* subversion/include/private/svn_opt_private.h,
  subversion/libsvn_subr/opt.c
  (svn_opt__split_arg_at_peg_revision): Don't throw an error when the path
    portion of the target is empty.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635118 13f79535-47bb-0310-9956-ffa450edef68