When using svn with revisions in the URL, corretly handly when they are deleted

If you have a SVN path that's deleted in a later revision, you need to
include the revision number in the URL to fetch so that SVN will send
you the code at that revision.

We had a problem with pip installing from a svn URL (i.e. ``svn+svn://example.com/path/to/code@100``), since the SVN server was now at version 150, and in version 140 we had deleted the directory ``/path/to/code``. In theory everything should still work, because at revision 100, the directory was still there. However pip wasn't able to make the URLs in the correct format.

The SVN book talks about this: http://svnbook.red-bean.com/en/1.7/svn-book.html#svn.advanced.pegrevs