SSH and host key verification

[smashwilson]

SSH remote operations fail when the host key is not already present in the known_hosts file:

Host key verification failed.
fatal: Could not read from remote repository.
Please make sure you have the correct access rights
and the repository exists.

Especially on Windows, where ~/.ssh/known_hosts is not a common file to have, this causes Problems ™️. I can think of a few ways we could potentially tackle this:

1. Find a way to hook the host key verification check and allow the user to confirm or reject it. I'm a little surprised it didn't use our SSH_ASKPASS handler, honestly.
2. Automatically trust-on-first-use. Maybe bootstrap with some common known hosts?
3. Integrate explicitly with pageant/putty and any other popular SSH clients. At least make some effort to connect with them if they're present.