Fix an issue with repository credential migration for older repositories

Summary: See IRC. It's possible to have a functional repository with the SSH username only in the URL. Look there if the username property isn't set. These should all be older repostiories.

Test Plan: Did a `--force --apply` upgrade.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D7665