[PATCH] add test for exporting specific revision with root-relative externals

The attached patch adds a new test, currently marked as XFail, for an
issue I recently discovered and posted about over on the users list
("Root-relative svn:externals result in error exporting a specific
revision"). In a nutshell, if you are exporting from a working copy with
the -r option, and the revision you are exporting has one or more
svn:externals with ^/ syntax, svn will exit with an assertion failure
while it is exporting the externals.

The log message is at the top of the patch.

Doug