Like other commands where a peg revision is not meaningful, make 'svn
export' error out if there is any non-empty peg specifier on the target
path, rather than just ignoring it.

This fixes one more case that was missed in r1196005 and r1196050. Related
to issue #3416 <http://subversion.tigris.org/issues/show_bug.cgi?id=3416>
"Cannot add or commit 'dir/@file.txt'" and to issue #3651
<http://subversion.tigris.org/issues/show_bug.cgi?id=3651> "svn copy does
not eat peg revision within copy target path".

* subversion/svn/export-cmd.c
  (svn_cl__export): Reject any non-empty peg revision specifier on the target.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635085 13f79535-47bb-0310-9956-ffa450edef68