Remove sensitivity to leading double slashes

For whatever reason, I ended up with a URL that looked like "ssh://git@github.com//org/repo.git". The double slash was causing `owner` to be "repo" and `name` to be nil. This lead to situations like this:

    ⇒ hub pull-request -b release
    Error creating pull request: Not Found (HTTP 404)
    Are you sure that github.com//repo exists?

Cleaned up the code to trim leading slashes.