Handle Subversion SSH on nonstandard ports

Summary:
Fixes T11203. Subversion handling of SVN_SSH commands requires some additional finesse for nonstandard remote SSH ports.

We get domain.com:port in the command, so parse it out if it's present.

Test Plan: @enckse confirmed this locally in T11203.

Reviewers: chad

Reviewed By: chad

Subscribers: enckse

Maniphest Tasks: T11203

Differential Revision: https://secure.phabricator.com/D16172