Fix path concatenation to prevent leading double slash
Fix path concatenation to prevent leading double slash

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.

Testing Done:
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

Reviewed at https://reviews.reviewboard.org/r/6962/

datjwu/rbtools,davidt/rbtools,haosdent/rbtools,beol/rbtools,beol/rbtools,halvorlu/rbtools,davidt/rbtools,halvorlu/rbtools,haosdent/rbtools,reviewboard/rbtools,reviewboard/rbtools,haosdent/rbtools,reviewboard/rbtools,datjwu/rbtools,datjwu/rbtools,davidt/rbtools,beol/rbtools,halvorlu/rbtools