Fix Subversion repositories on hosting services.
The Subversion code expects to be able to access Subversion repositories

directly. This works fine, except when accessing through a hosting

service and needing a login/password. The Subversion code expects

repository.username and repository.password to be filled in.
There's now a Repository.get_credentials() function that fetches the

username/password from either the repository or the hosting service (if

it otherwise stores the password). This allows the Subversion support to

log in appropriately.
There are other SCMTools that also make use of the username/password, so

I've updated all call sites to use get_credentials. This will allow us

to do stuff like use decrypt_password in a central place in the future.

Unit tests pass.
Used this with the new Unfuddle support, fixing SVN access.