Check SSH_AUTH_SOCK to determine existence of ssh-agent

[talpr]

See discussion in #81.

[martinvonz]

Thanks for looking into this. I had just used SSH_AGENT_PID without thinking about it before.

Could you add mention of it in the changelog? In particular, it may be useful to say something about who may have run into the problem before. I'm hoping it may help convince some users who have tried jj before and just gave up because authentication didn't work for them. Perhaps something like this:

• ssh-agent is now detected even if $SSH_AGENT_PID is not set (as long as $SSH_AUTH_SOCK is set). This should help at least macOS users where ssh-agent is launched by default and only $SSH_AUTH_SOCK is set.

Actually, is that the case for you? That you're on macOS and have not manually started ssh-agent, I mean?

[talpr]

Done.

Actually, is that the case for you? That you're on macOS and have not manually started ssh-agent, I mean?

Yup, basically. TBH, I don't remember ever starting the agent myself, not on Macs and not on Linux machines, but I have no idea what magic might have been running it for me all this time. 😅