[PATCH] add ability to export a specific revision when a working copy path is supplied

The attached patch fixes an assertion failure when exporting from a
working copy with the -r option when you have svn:externals that use the
root-relative ^/ syntax, and updates the
export_revision_with_root_relative_external test I recently wrote that
checks for this situation.

I'm completely new to the Subversion codebase and would appreciate
feedback on anything I should have done differently for this fix. I'm
unsure if this is the correct fix for this problem, but it definitely
makes the test pass and works properly for exporting in the situation
where I originally found this problem. I've tested with "make check"
and everything passes. Daniel Shahaf very graciously nudged me in the
direction of writing this patch myself -- thanks for all your help Daniel!

The patch contains the log message, which explains the change in a bit
more detail.

Doug