Weaken the type requirement on the instanceof used in the git-client plugin to only rely on the object being a StandardUsernamePasswordCredentials.

The interface is both necessary and sufficient for the APIs used.

This is morally equivalent to how the private key logic is already using the SSHUserPrivateKey interface vs. a specific implementation.
Merge pull request #32 from mattmoor/master

Weaken the type requirement on the instanceof used in the git-client plu...