Check if SSH_AUTH_SOCK env var is set before trying to use the SSH agent.

[lapfelix]

Fixes the cloning test through ssh (GCSingleCommitRepositoryTests's testClone) on computers that don't have a correctly setup ssh agent, like mine 🥴 (as the keys in ~/.ssh will be used instead, and that's going to work as long as they're not protected by a passphrase).

If SSH_AUTH_SOCK isn't there, it will fail later on anyway when libgit2 uses libssh2.

Should also make ssh agent workarounds like this one unnecessary: #161 (comment)

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[lucasderraugh]

@lapfelix Being relatively unfamiliar with this portion of the codebase, does libssh2 not support accessing passphrased sah keys?

[lapfelix]

hmm it would definitely be possible to support passphrased ssh keys, and GitUp could save the passphrase in the keychain.

(In theory though the ssh agent should "just work" but in practice, I'm not sure why it's not super reliable and why it sometimes needs to be kickstarted after a reboot)

[lucasderraugh]

I guess I'm trying to figure out what this particular PR is trying to improve. Wouldn't this environment variable be set as long as ssh-agent was running? Is this just meant to short circuit the case where it isn't?

[lapfelix]

Yes basically I didn't find any straightforward way to retry calling the _CredentialsCallback, but if the ssh-agent isn't running, the environment variable won't be set, so we don't even need to try the ssh-agent, we know it'll fail.

This PR helps cases where the ssh agent isn't running, but maybe there's a more "correct" solution to the problem. Like I wonder when line 455 ever executed, for instance