Configure ssh.clientOptions using environment variables. Fixes go-git#411.

[smveloso]

I stumbled upon #411 (which is actually caused by golang/go#29286) and I did not like the idea of relying solely on editing the "known_hosts" file as a workaround. So I tried to see if I could set ssh client parameters in code.

Although a work in progress, I think this might be a worthy (very small) contribution.

I chose to do it with environment variables at first to minimize changes in existing code, but I also think this approach may be better suited for applications that use go-git in container environments (as opposed to a configuration file) and ones that deal with a limited set of remote repositories.

Maybe a more general approach might be reading a standard ssh client (ssh_config) file; the file format already allows setting different parameters for different remote hosts. However, in containers/kubernetes scenarios, creating a file and making it available as a volume seems to be too much, unless you really need many different "profiles" in your use case.

[smveloso]

I think the other approaches would be:

1. Add a field to every struct that is used as argument in functions that might result in ssh operations So, for example, CloneOptions would have an "SSHClientOptions" field.

2. Wait for golang's ssh library to implement auto-configuration (set "HostKeyAlgorithms" according to what is available in the known_hosts file(s)).

3. Implement auto-configuration directly in go-git.

Option "3" unfortunately won't be able to leverage code from golang's "x/crypto/ssh/knownhosts" package, because the relevant parts that process "known_hosts" files are not public.

[mcuadros]

Fixed by #548