Fix for "Brown Out" when trying to connect to Microsoft DevOps Repos

[flocko-motion]

Microsoft introduced a "brownout" for SSH-RSA based authenticaction. In the future, only RSA-SHA2-256 and RSA-SHA2-512 are accepted. This does make a lot of sense, as the SHA-1 algorithm used for SSH-RSA authentication is not considered state of the art any more.

Details on SSH-RSA deprecaction:
https://devblogs.microsoft.com/devops/ssh-rsa-deprecation/

This patch changes the configuration of the SSH client to use RSA-SHA2-256 and RSA-SHA2-512 only. It's not advised to keep using SSH-RSA.

The fix caused by go-git based program to work again.

I'm very sorry for not providing a unit-test and detailled bug reports with this fix. It does work - I'm using it in production. It's also very tiny, so you might be able to accept it from a simple review.

Otherwise, please feed back to me and I'll try to provide more material, as soon as I find the time.

[pjbgf]

@flocko-motion thanks for your contribution. The changes could break existing setups, so ideally we need to provide a way for go-git users to change the HostKeyAlgorithms.