ssh authenticate

[wiegelmann]

fixes #934 and #1023

[MichaelMure]

Hi, thank you for doing this.

A few things:

• as we have seen with the recent XY backdoor, it's important for that kind of code to be cristal clear: could you comment in the body of those function about what you are doing and why?
• is that a workaround for a go-git issue? If so, could you mark it clearly, ideally with a link to an upstream issue?

[wiegelmann]

Hi, according to go-git/go-git#218 go-git does not load public keys from ~/.ssh, meaning you need an active ssh-agent running otherwise you will get errors like:

• Linux: error creating SSH agent: "SSH agent requested but SSH_AUTH_SOCK not-specified"
• Mac: ssh: handshake failed: ssh: unable to authenticate, attempted methods [none publickey], no supported methods remain
• Windows: error creating SSH agent: "SSH agent requested, but could not detect Pageant or Windows native SSH agent"

If the ssh-agent is running and contains valid keys, everything is fine. If the ssh-agent has invalid keys or is not running, we try to find public keys in ~/.ssh and retry fetch/push again.

Tested this PR on Linux, Mac with the following scenarios:

• ssh-agent running with valid and invalid keys in ~/.ssh
• ssh-agent not running with valid keys in ~/.ssh

On Windows fetch/pull fails with and without ssh-agent. With this PR it works with valid keys in ~/.ssh.