In SVNClient.convert_to_absolute_paths() when concatenating paths when the --repository-url option is specified, it was possible to end up with a leading double slash when repository_info.base_path was '/'.  Switch to posixpath.join() to correctly concatenate paths for all cases, with platform independent Unix-style slashes.

Tested on both Linux and Windows.

Tested where base_path = '/':
Old output: //file.txt
New output: /file.txt

Tested where base_path = '/foo':
Old output: /foo/file.txt
New output: /foo/file.txt