Fix 'rbt close' to not require a working directory if the server is specified.

Because of rbt's post-review heritage, it often requires a working directory,
even if all it uses it for is to detect the Review Board server path. This
change fixes 'rbt close' to skip the SCM detection if a server is specified
(either via the command-line or the home directory ~/.reviewboardrc).

Testing Done:

Closed https://reviews.reviewboard.org/r/5464/ by running
    rbt close --server=https://reviews.reviewboard.org/ 5464 --description="Closing review with rbt close --server=https://reviews.reviewboard.org/ 5464"

Bugs closed: 3249

Reviewed at https://reviews.reviewboard.org/r/5465/