Expose stock user/pass credential utility

Populating a `git_cred **` from managed code in libgit2sharp is hard. Let's go shopping!

This PR takes a function that was used for the clar tests, bullet-proofs it a bit, and exposes it as a stock authentication method usable as a `git_cred_acquire_cb`. 

*[Also, a couple of the online clone tests weren't renamed when clar 2.0 happened. Ssh.]*